cache: remove branch revision file before rewriting the branch name file

New branch names are usually appended to the branch name file. If that fails or
the file has been modified by another process, it is rewritten. That left a
small opportunity that there could be references to non-existent entries in the
file while it was rewritten.

To avoid that, remove the revision branch cache file with the references to the
branch name file before rewriting the branch name file. Worst case, when
interrupted at the wrong time, the cache will be lost and rebuilt next time.

It is unknown if this fixes a real problem that ever happened.

mercurial/branchmap.py

@@ -461,6 +461,8 b' class revbranchcache(object):'
                         self._rbcnamescount = 0
                         self._rbcrevslen = 0
                 if self._rbcnamescount == 0:
+                    # before rewriting names, make sure references are removed
+                    repo.vfs.unlinkpath(_rbcrevs, ignoremissing=True)
                     f = repo.vfs.open(_rbcnames, 'wb')
                 f.write('\0'.join(encoding.fromlocal(b)
                                   for b in self._names[self._rbcnamescount:]))