add minimal coloring for gitcommit highlighting

colors/gruvbox.vim

@@ -648,6 +648,12 @@ else
 	call s:HL('GitGutterChangeDelete', 'aqua', g:gruvbox_sign_column, 'inverse')
 endif
 
+" }}}
+" gitcommit highlighting "{{{
+
+call s:HL('gitcommitSelectedFile', 'green')
+call s:HL('gitcommitDiscardedFile', 'red')
+
 " }}}
 " Signify: {{{