Make buffer-file-name absolute

As by documentation, the value of buffer-file-name should be
an *absolute* file-name.
Use appropriate function to obtain a relative file name
Don't use the shell to extract top-level
Use appropriate function to append trailing slash
Drop the redundant dependency on s.el
Simplify git-timemachine-kill-revision

Directly fill the kill ring with "kill-new" instead of going through a
temporary buffer.
Simplify git-timemachine-show-revision

Directly call "git" and insert the output into the target buffer instead
of running "git" through the shell, obtaining the output as string and
inserting it.
Simplify git-timemachine--revisions
Use appropriate let variant

The bindings are not recursive anymore
Merge pull request #8 from lunaryorn/misc-improvements

Miscellaneous improvements