feat(config): add gitcommit highlight defaults

There are some more changes which should be made in order to maintain
consistency across environments. Directory is now used for file names,
and SpecialComment is used for the headers.

colors/highlite.vim

@@ -664,7 +664,9 @@ local highlight_groups = {
 	yamlKey = 'Label',
 
 	--[[ 4.3.34. Git ]]
+	gitcommitHeader = 'SpecialComment',
 	gitcommitOverFlow = 'Error',
+	gitcommitSelectedFile = 'Directory',
 	gitcommitSummary  = 'Title',
 	gitconfigAssignment = 'String',
 	gitconfigEscape = 'SpecialChar',