Add the Mercurial test repository cache directory to .gitignore.
Review Request #1779 — Created Nov. 7, 2019 and updated
When generating test data using manage.py fill-database, the directory reviewboard/scmtools/testdata/hg_repo/.hg/wcache/ was being consistently generated. It was added to the .gitignore. Reviewed at https://reviews.reviewboard.org/r/10686/