Git-Support: Fixed working directory issue.

- The buffers now keep track of the working directory they were last updated from.
- Minor corrections.
Merge remote-tracking branch 'origin/master'
Git-Support: Improved syntax highlighting and folding.

- New 'foldtext' function, which produces custom text for diff, log, status ...
- New syntax for commit messages, marks lines which are to long.
- Separate syntax file for diff, used by log and status.
- Syntax categories now have a fallback for highlighting using Vim's own categories.
- Documentation.