revbranchcache: move out of branchmap onto localrepo Previously the revbranchcache was a field inside the branchmap. This is bad for a couple reasons: 1) There can be multiple branchmaps per repo (one for each filter level). There can only be one revbranchcache per repo. In fact, a revbranchcache could only exist on a branchmap that was for the unfiltered view, so you could have branchmaps exist for which you couldn't have a revbranchcache. It was funky. 2) The write lifecycle for the revbranchcache is going to be different from the branchmap (branchmap is greedily written early on, revbranchcache should be lazily computed and written). This patch moves the revbranchcache to live as a field on the localrepo (alongside self._branchmap). This will allow us to handle it's lifecycle differently, which will let us move it to be lazily computed in future patches.

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.