feat: highlight commit buffers (#112)

closes #109 

`:G commit` buffers are now highlighted as follows:

<img width="556" height="502" alt="image"
src="https://github.com/user-attachments/assets/4248dc42-c151-4ec8-b4b7-43b6fe919749"
/>

Co-authored-by: Barrett Ruth <br@barrettruth.com>

plugin/diffs.lua

@@ -6,7 +6,7 @@ vim.g.loaded_diffs = 1
 require('diffs.commands').setup()
 
 vim.api.nvim_create_autocmd('FileType', {
-  pattern = { 'fugitive', 'git' },
+  pattern = { 'fugitive', 'git', 'gitcommit' },
   callback = function(args)
     local diffs = require('diffs')
     if args.match == 'git' and not diffs.is_fugitive_buffer(args.buf) then