contrib/vim: add syntax highlighting file for commits

Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

contrib/vim/README

+To syntax highlight git's commit messages, you need to:
+  1. Copy syntax/gitcommit.vim to vim's syntax directory:
+     $ mkdir -p $HOME/.vim/syntax
+     $ cp syntax/gitcommit.vim $HOME/.vim/syntax
+  2. Auto-detect the editing of git commit files:
+     $ cat >>$HOME/.vimrc <<'EOF'
+     autocmd BufNewFile,BufRead COMMIT_EDITMSG set filetype=gitcommit
+     EOF

contrib/vim/syntax/gitcommit.vim

+syn region gitLine start=/^#/ end=/$/
+syn region gitCommit start=/^# Updated but not checked in:$/ end=/^#$/ contains=gitHead,gitCommitFile
+syn region gitHead contained start=/^#   (.*)/ end=/^#$/
+syn region gitChanged start=/^# Changed but not updated:/ end=/^#$/ contains=gitHead,gitChangedFile
+syn region gitUntracked start=/^# Untracked files:/ end=/^#$/ contains=gitHead,gitUntrackedFile
+
+syn match gitCommitFile contained /^#\t.*/hs=s+2
+syn match gitChangedFile contained /^#\t.*/hs=s+2
+syn match gitUntrackedFile contained /^#\t.*/hs=s+2
+
+hi def link gitLine Comment
+hi def link gitCommit Comment
+hi def link gitChanged Comment
+hi def link gitHead Comment
+hi def link gitUntracked Comment
+hi def link gitCommitFile Type
+hi def link gitChangedFile Constant
+hi def link gitUntrackedFile Constant