Add blank .gitignore files in the git-repo refs/ and tags/ directories

These directories are (currently) empty, which means git won't commit them and
unit tests will break. Blank .gitignore files don't seem to cause unwanted
behavior but does let them be part of the repo.

Testing done: Ran unit tests.
Remove an unused method.

The _svn_add_file_commit method wasn't used, and in fact causes undesirable
behavior because it writes data into the testdata/svn-repo directory (since a
commit in svn is a push). Trivial cleanup.