Skip a test in test_filediff_creator if Mercurial is not installed.

Review Request #109 — Created Sept. 5, 2025 and updated — Latest diff uploaded

Information

Review Board
ab783e1...

Reviewers

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/