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/git.lua

@@ -0,0 +1,66 @@
+local M = {}
+
+---@param filepath string
+---@return string?
+function M.get_git_dir(filepath)
+  local dir = vim.fn.fnamemodify(filepath, ':h')
+  local result = vim.fn.systemlist({ 'git', '-C', dir, 'rev-parse', '--git-dir' })
+  if vim.v.shell_error ~= 0 then
+    return nil
+  end
+  local git_dir = result[1]
+  if not git_dir then
+    return nil
+  end
+  if vim.startswith(git_dir, '/') then
+    return git_dir
+  end
+  return vim.fn.fnamemodify(dir .. '/' .. git_dir, ':p'):gsub('/+$', '')
+end
+
+---@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