vim-git: add support for git format-patch messages

Taking the diff.vim and mail.vim settings, create a git-format-patch
filetype which uses settings to show syntax highlighted diffs as well as
proper mail message format. Include a text width so that correct
paragraph formatting is also used. The primary purpose for this change is for use of the --annotate and --cover-letter options of git-send-email for editing a format-patch file.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>