add filetype for mercurial checkins

```
au BufNewFile,BufRead hg-editor-*.txt setf hgcommit

And a search found this syntax file:
http://code.delroth.net/configs/src/ca2adbde207b/.vim/syntax/hgcommit.vim
```

Original issue reported on code.google.com by `colossus...@gmail.com` on 7 Aug 2011 at 3:03
