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.

contrib/perf.py

@@ -2347,6 +2347,20 b' def perfbranchmapupdate(ui, repo, base=('
         baserepo = repo.filtered(b'__perf_branchmap_update_base')
         targetrepo = repo.filtered(b'__perf_branchmap_update_target')
 
+        # try to find an existing branchmap to reuse
+        subsettable = getbranchmapsubsettable()
+        candidatefilter = subsettable.get(None)
+        while candidatefilter is not None:
+            candidatebm = repo.filtered(candidatefilter).branchmap()
+            if candidatebm.validfor(baserepo):
+                filtered = repoview.filterrevs(repo, candidatefilter)
+                missing = [r for r in allbaserevs if r in filtered]
+                base = candidatebm.copy()
+                base.update(baserepo, missing)
+                break
+            candidatefilter = subsettable.get(candidatefilter)
+        else:
+            # no suitable subset where found
         base = branchmap.branchcache()
         base.update(baserepo, allbaserevs)