vim: add git-diff.vim in README, make syntax work w/o the script

vim/README

@@ -3,15 +3,22 @@ through vim.
 
 NOTE: this is a work in progress.  Patches welcome. <bart@jukie.net>
 
+The vim interface makes use of the git-diff.vim syntax file which is
+available from
+    http://github.com/motemen/git-vim/blob/master/syntax/git-diff.vim
+
+
 To install:
         make install
 
+
 To run:
         vim -c ':NotMuch'
 
     from vim:
         :NotMuch
 
+
 Buffer types:
     [notmuch-folders]
         Folder list, or technically a list of saved searches.

vim/syntax/notmuch-show.vim

@@ -11,7 +11,9 @@ syntax match   nmShowMsgHeadVal /^\([^:]\+: \)\@<=.*/ contained
 
 syntax cluster nmShowMsgBody contains=@nmShowMsgBodyMail,@nmShowMsgBodyGit
 syntax include @nmShowMsgBodyMail syntax/mail.vim
-syntax include @nmShowMsgBodyGit syntax/git-diff.vim
+
+" git-diff.vim marks up diffs in emails, see README for details
+silent! syntax include @nmShowMsgBodyGit syntax/git-diff.vim
 
 highlight nmShowMsgDescWho term=reverse cterm=reverse gui=reverse
 highlight link nmShowMsgDescDate Type