remove vim syntax highlighting in favor of upstream

As of version 7.2, vim ships with its own syntax
highlighting for git commit messages, which is:

  1. more comprehensive in splitting up the various
     components of the file

  2. in accordance with the usual vim behavior for syntax
     highlighting (e.g., respecting b:current_syntax)

  3. presumably better maintained (I have not been using
     what's in git's contrib/ directory for some time in
     favor of the upstream version)

Furthermore, vim upsream also provides syntax highlighting
for other git filetypes (gitconfig, rebase, send-email).

This patch gets rid of our local version and just points
interested parties to the upstream version.

The code for auto-detecting filetypes is taken from vim's
runtime/filetype.vim.
Signed-off-by: NSZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

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
+Syntax highlighting for git commit messages, config files, etc. is
+included with the vim distribution as of vim 7.2, and should work
+automatically.
+
+If you have an older version of vim, you can get the latest syntax
+files from the vim project:
+
+  http://vim.svn.sourceforge.net/viewvc/vim/trunk/runtime/syntax/git.vim
+  http://vim.svn.sourceforge.net/viewvc/vim/trunk/runtime/syntax/gitcommit.vim
+  http://vim.svn.sourceforge.net/viewvc/vim/trunk/runtime/syntax/gitconfig.vim
+  http://vim.svn.sourceforge.net/viewvc/vim/trunk/runtime/syntax/gitrebase.vim
+  http://vim.svn.sourceforge.net/viewvc/vim/trunk/runtime/syntax/gitsendemail.vim
+
+To install:
+
+  1. Copy these files to vim's syntax directory $HOME/.vim/syntax
+  2. To auto-detect the editing of various git-related filetypes:
+	$ cat >>$HOME/.vim/filetype.vim <<'EOF'
+	autocmd BufNewFile,BufRead *.git/COMMIT_EDITMSG    setf gitcommit
+	autocmd BufNewFile,BufRead *.git/config,.gitconfig setf gitconfig
+	autocmd BufNewFile,BufRead git-rebase-todo         setf gitrebase
+	autocmd BufNewFile,BufRead .msg.[0-9]*
+		\ if getline(1) =~ '^From.*# This line is ignored.$' |
+		\   setf gitsendemail |
+		\ endif
+	autocmd BufNewFile,BufRead *.git/**
+		\ if getline(1) =~ '^\x\{40\}\>\|^ref: ' |
+		\   setf git |
+		\ endif
+	EOF

contrib/vim/syntax/gitcommit.vim

-syn region gitLine start=/^#/ end=/$/
-syn region gitCommit start=/^# Changes to be committed:$/ 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