remove vim syntax highlighting in favor of upstream

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: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>