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/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