Bug fix for one-liner diffs

[thoward]

This change fixes unified diff parsing to work correctly for one-liner files. Previously, the parser was giving no output in those situations.

Steps to reproduce:

$ mkdir test_repo
$ cd test_repo
$ git init
$ echo "Adding a one-line file." > oneline.txt
$ git commit -am "Add a file"
$ echo "Changed a one-line file." > oneline.txt
$ git commit -am "Changed a file"
$ git rm oneline.txt
$ git commit -m "Delete a file"
$ git log -p

Grab any of the diffs in that git log. None of them would parse correctly. This is because the comparison in parse_unified_diff of the line offsets r and i were bypassing at 0 when no old_len/new_len value was available.

I've included unit tests and casefile fixtures for this change. This does not break any other unit tests, although there are an unrelated set of 3 tests which are currently broken (as described in issue #1).

Separately, I did look into to those failing tests and I can see that these are intentional test failures, because they are testing not-yet-implemented features. Yay TDD!

[cscorley]

Great catch and PR! Thanks so much.