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.

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)