branchcache: add more test for the logic around obsolescence and branch heads While working on branch-cache-v3, we noticed some ambiguity in the filtered+obsolete hash. However this was only caught by a rebase test by chance. It seems important to explicitly tests these cases.

local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed