perf: start from an existing branchmap if possible If the --base set if a superset of one of the cached branchmap, we should use as a starting point. This greatly help the overall runtime of `hg perfbranchmapupdate` For example, for a repository with about 500 000 revisions, using this trick make the command runtime move from about 200 second to about 10 seconds. A 20x gain.