Git-Support: Added more Git commands.

- Added 'blame', 'mv', 'show' and 'tag'.
- Improved 'commit':
  Redirecting the output of 'commit --dry-run ...' into a buffer.
- Improved 'stash':
  Redirecting the output of 'stash list' and 'stash show' into buffers.
- Minor changes.
- Updated the documentation.