Add function to blame current buffer for specified revision

Provide an interactive function similar to `mo-git-blame-current' that
also lets the user specify the starting revision to be used.
Merge pull request #8 from mgalgs/blame-current-for-rev

Add function to blame current buffer for specified revision
Correctly handle symlinked files.

If a file is accessed via a symlink the relative path
can be in a format not usable for git blame.

/gitroot/Some/File -> /data/gitroot/Some/File

If the file is opend via the symlink (buffer-file-name)
would return "/gitroot/Some/File" but top-dir would be
"/data/gitroot/Some/File". This then leads to a relative
file path like "../../../data/gitroot/Some/File" instead
of just "Some/File" which cannot be handled by git blame.
Support passing extra arguments to git blame.

This supports cases such as "-w", and addresses
mbunkus/mo-git-blame#3.
Merge pull request #10 from nyergler/master

Support passing extra arguments to git blame.
Merge pull request #9 from rcj/master

Correctly handle symlinked files.