feat: add :Gdiff, :Gvdiff, :Ghdiff commands for unified diff view

Compares current buffer against any git revision (default HEAD), opens result
with full diffs.nvim syntax highlighting. Follows fugitive convention:
:Gdiff/:Gvdiff open vertical split, :Ghdiff opens horizontal split.

lua/diffs/commands.lua

@@ -0,0 +1,114 @@
+local M = {}
+
+local git = require('diffs.git')
+local dbg = require('diffs.log').dbg
+
+---@param old_lines string[]
+---@param new_lines string[]
+---@param old_name string
+---@param new_name string
+---@return string[]
+local function generate_unified_diff(old_lines, new_lines, old_name, new_name)
+  local old_content = table.concat(old_lines, '\n')
+  local new_content = table.concat(new_lines, '\n')
+
+  local diff_fn = vim.text and vim.text.diff or vim.diff
+  local diff_output = diff_fn(old_content, new_content, {
+    result_type = 'unified',
+    ctxlen = 3,
+  })
+
+  if not diff_output or diff_output == '' then
+    return {}
+  end
+
+  local diff_lines = vim.split(diff_output, '\n', { plain = true })
+
+  local result = {
+    'diff --git a/' .. old_name .. ' b/' .. new_name,
+    '--- a/' .. old_name,
+    '+++ b/' .. new_name,
+  }
+  for _, line in ipairs(diff_lines) do
+    table.insert(result, line)
+  end
+
+  return result
+end
+
+---@param revision? string
+---@param vertical? boolean
+function M.gdiff(revision, vertical)
+  revision = revision or 'HEAD'
+
+  local bufnr = vim.api.nvim_get_current_buf()
+  local filepath = vim.api.nvim_buf_get_name(bufnr)
+
+  if filepath == '' then
+    vim.notify('[diffs.nvim]: cannot diff unnamed buffer', vim.log.levels.ERROR)
+    return
+  end
+
+  local rel_path = git.get_relative_path(filepath)
+  if not rel_path then
+    vim.notify('[diffs.nvim]: not in a git repository', vim.log.levels.ERROR)
+    return
+  end
+
+  local old_lines, err = git.get_file_content(revision, filepath)
+  if not old_lines then
+    vim.notify('[diffs.nvim]: ' .. (err or 'unknown error'), vim.log.levels.ERROR)
+    return
+  end
+
+  local new_lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+
+  local diff_lines = generate_unified_diff(old_lines, new_lines, rel_path, rel_path)
+
+  if #diff_lines == 0 then
+    vim.notify('[diffs.nvim]: no diff against ' .. revision, vim.log.levels.INFO)
+    return
+  end
+
+  local diff_buf = vim.api.nvim_create_buf(false, true)
+  vim.api.nvim_buf_set_lines(diff_buf, 0, -1, false, diff_lines)
+  vim.api.nvim_set_option_value('buftype', 'nofile', { buf = diff_buf })
+  vim.api.nvim_set_option_value('bufhidden', 'wipe', { buf = diff_buf })
+  vim.api.nvim_set_option_value('modifiable', false, { buf = diff_buf })
+  vim.api.nvim_set_option_value('filetype', 'diff', { buf = diff_buf })
+  vim.api.nvim_buf_set_name(diff_buf, 'diffs://' .. revision .. ':' .. rel_path)
+
+  vim.cmd(vertical and 'vsplit' or 'split')
+  vim.api.nvim_win_set_buf(0, diff_buf)
+
+  dbg('opened diff buffer %d for %s against %s', diff_buf, rel_path, revision)
+
+  vim.schedule(function()
+    require('diffs').attach(diff_buf)
+  end)
+end
+
+function M.setup()
+  vim.api.nvim_create_user_command('Gdiff', function(opts)
+    M.gdiff(opts.args ~= '' and opts.args or nil, false)
+  end, {
+    nargs = '?',
+    desc = 'Show unified diff against git revision (default: HEAD)',
+  })
+
+  vim.api.nvim_create_user_command('Gvdiff', function(opts)
+    M.gdiff(opts.args ~= '' and opts.args or nil, true)
+  end, {
+    nargs = '?',
+    desc = 'Show unified diff against git revision in vertical split',
+  })
+
+  vim.api.nvim_create_user_command('Ghdiff', function(opts)
+    M.gdiff(opts.args ~= '' and opts.args or nil, false)
+  end, {
+    nargs = '?',
+    desc = 'Show unified diff against git revision in horizontal split',
+  })
+end
+
+return M

lua/diffs/git.lua

@@ -0,0 +1,48 @@
+local M = {}
+
+---@param filepath string
+---@return string?
+function M.get_repo_root(filepath)
+  local dir = vim.fn.fnamemodify(filepath, ':h')
+  local result = vim.fn.systemlist({ 'git', '-C', dir, 'rev-parse', '--show-toplevel' })
+  if vim.v.shell_error ~= 0 then
+    return nil
+  end
+  return result[1]
+end
+
+---@param revision string
+---@param filepath string
+---@return string[]?, string?
+function M.get_file_content(revision, filepath)
+  local repo_root = M.get_repo_root(filepath)
+  if not repo_root then
+    return nil, 'not in a git repository'
+  end
+
+  local rel_path = vim.fn.fnamemodify(filepath, ':.')
+  if vim.startswith(filepath, repo_root) then
+    rel_path = filepath:sub(#repo_root + 2)
+  end
+
+  local result = vim.fn.systemlist({ 'git', '-C', repo_root, 'show', revision .. ':' .. rel_path })
+  if vim.v.shell_error ~= 0 then
+    return nil, 'failed to get file at revision: ' .. revision
+  end
+  return result, nil
+end
+
+---@param filepath string
+---@return string?
+function M.get_relative_path(filepath)
+  local repo_root = M.get_repo_root(filepath)
+  if not repo_root then
+    return nil
+  end
+  if vim.startswith(filepath, repo_root) then
+    return filepath:sub(#repo_root + 2)
+  end
+  return vim.fn.fnamemodify(filepath, ':.')
+end
+
+return M

lua/diffs/highlight.lua

@@ -28,8 +28,8 @@ local function highlight_text(bufnr, ns, hunk, col_offset, text, lang)
   local extmark_count = 0
   local header_line = hunk.start_line - 1
 
-  for id, node, _ in query:iter_captures(trees[1]:root(), text) do
-    local capture_name = '@' .. query.captures[id]
+  for id, node, metadata in query:iter_captures(trees[1]:root(), text) do
+    local capture_name = '@' .. query.captures[id] .. '.' .. lang
     local sr, sc, er, ec = node:range()
 
     local buf_sr = header_line + sr
@@ -37,11 +37,13 @@ local function highlight_text(bufnr, ns, hunk, col_offset, text, lang)
     local buf_sc = col_offset + sc
     local buf_ec = col_offset + ec
 
+    local priority = lang == 'diff' and (tonumber(metadata.priority) or 100) or 200
+
     pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_sr, buf_sc, {
       end_row = buf_er,
       end_col = buf_ec,
       hl_group = capture_name,
-      priority = 200,
+      priority = priority,
     })
     extmark_count = extmark_count + 1
   end
@@ -105,8 +107,8 @@ local function highlight_treesitter(bufnr, ns, hunk, code_lines, col_offset)
   col_offset = col_offset or 1
 
   local extmark_count = 0
-  for id, node, _ in query:iter_captures(trees[1]:root(), code) do
-    local capture_name = '@' .. query.captures[id]
+  for id, node, metadata in query:iter_captures(trees[1]:root(), code) do
+    local capture_name = '@' .. query.captures[id] .. '.' .. lang
     local sr, sc, er, ec = node:range()
 
     local buf_sr = hunk.start_line + sr
@@ -114,11 +116,13 @@ local function highlight_treesitter(bufnr, ns, hunk, code_lines, col_offset)
     local buf_sc = sc + col_offset
     local buf_ec = ec + col_offset
 
+    local priority = lang == 'diff' and (tonumber(metadata.priority) or 100) or 200
+
     pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_sr, buf_sc, {
       end_row = buf_er,
       end_col = buf_ec,
       hl_group = capture_name,
-      priority = 200,
+      priority = priority,
     })
     extmark_count = extmark_count + 1
   end

lua/diffs/log.lua

@@ -13,7 +13,7 @@ function M.dbg(msg, ...)
   if not enabled then
     return
   end
-  vim.notify('[diffs] ' .. string.format(msg, ...), vim.log.levels.DEBUG)
+  vim.notify('[diffs.nvim]: ' .. string.format(msg, ...), vim.log.levels.DEBUG)
 end
 
 return M