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.

plugin/diffs.lua

@@ -3,6 +3,8 @@ if vim.g.loaded_diffs then
 end
 vim.g.loaded_diffs = 1
 
+require('diffs.commands').setup()
+
 vim.api.nvim_create_autocmd('FileType', {
   pattern = { 'fugitive', 'git' },
   callback = function(args)