Uncommented tester code in diff.rb.

There existed some code in diff.rb I was using for testing purposes. I
commented it out before publishing it. However, I have found it to be
more convenient to put it into a separate method; executed whenever the
file is run.
Git now ignores make files.
Added initial code for branches.

Branches now have three attributes - name, heads, diffs. This will
probably change soon.
Added basic commit framework.

I wrote some code that generates a Commit instance (Commit.rb). The
Commit instance stores the differences between the previous file
("head") and the new file ("foot"); overwrites the contents of head with
the contents of foot.

The code I wrote in the zrc/ directory is really just a library for the
stuff in zn/ . In the interest of unambiguity, I will probably move the
zrc/ directory to libzn/ sometime in the near future.