feat: add :Gdiff command for unified diff against git revision

Compares current buffer against any git revision (default HEAD), opens result
in vsplit with full diffs.nvim syntax highlighting.

lua/diffs/commands.lua

@@ -0,0 +1,98 @@
+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_output = vim.diff(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
+function M.gdiff(revision)
+  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: buffer has no file', vim.log.levels.ERROR)
+    return
+  end
+
+  local rel_path = git.get_relative_path(filepath)
+  if not rel_path then
+    vim.notify('diffs: 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: ' .. (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: no changes', 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('vsplit')
+  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)
+  end, {
+    nargs = '?',
+    desc = 'Show unified diff against git revision (default: HEAD)',
+  })
+end
+
+return M