One of the unit tests in the FileDiffCreatorTests is pretty Mercurial

specific (testing the case of an hg diff without per-file revisions but

with a valid parent diff). Because we don't install Mercurial by

default, this test fails in a brand new virtualenv.
Testing Done:

Ran test suite without Mercurial installed and saw that the failing

test was now skipped.
Reviewed at https://reviews.reviewboard.org/r/14544/