Fix for the diff action in the commit window

Starting with mate 2.5 it will not read from standard input if the TM_DOCUMENT_UUID environment variable is set. This is unfortunate for commands launched by TextMate (where the variable is set) that use mate to pass data to TextMate (via mate’s stdin).

This fix is provisional until a new version of mate gets released.
Allow comments in commit message area
Require comments to start a line
Correctly determine working copy root when inside submodule.

This fixes error messages like "fatal: '/path/to/file' is outside repository" when using the bundle with git submodules.

`Git.path` assumed that the git dir would always be located directly below the working copy root. However, this is not (no longer?) true when working with submodules: The git dir of a submodule is nested inside the parent project’s git dir.

Using `git rev-parse --show-toplevel` (available since Git 1.7.0) figures this out correctly, even for submodules.

Similar incorrect code existed in `Compare Branches.tmCommand`. This was dead code however (`path` not being used anywhere afterwards), so I removed the line there completely.