localrepo: strip now incrementally updates the branchheads cache Destroying history via strip used to invalidate the branchheads cache, causing it to be regenerated the next time it is read. This is expensive in large repos. This change converts strip to pass info to localrepo.destroyed() to enable to it to incrementally update the cache, improving the performance of strip and other operations that depend on it (e.g., rebase). This change also strengthens a bit the integrity checking of the branchheads cache when it is read, by rejecting the cache if it has nodes in it that no longer exist.