Pull request vim - improve options

First of all, thank you for working on this fantastic project. I've began using it daily.

Now about the issue.
I noticed pull request message file is set to `gitcommit` file type. Here are the downsides of it:
 - textwidth is set to 72 - perfectly fine when writing actual commits, less so when writing a message that will be read solely on the web. Just for the reference, here is a dummy pull request with `tw=72` when [viewed on github](http://cl.ly/image/3F2g1p3M1y44). I'd just let github's designers worry about text width.
 - `DiffGitCached` command is present but broken [link](https://github.com/tpope/vim-git/blob/master/ftplugin/gitcommit.vim#L32).

This pull request I hope eliminates the above mentioned downsides, while keeping the benefits of nice syntax highlighting and `keywordprg` from git file type.