revbranchcache: add test for when the cache is not writable The revbranchecache code already handled the case when the cache file wasn't writable, but let's add a test as well so future changes don't regress this.

patch.trydiff: add a docstring It took me a bit to figure out what this function actually does.

keyword: monkeypatch patch.diff more generically This function doesn't need access to any of the args or kwargs, so make the monkeypatching more robust. (In upcoming patches we'll introduce another argument to patch.diff, and this function would break if it weren't for this patch.)