branchcache: add a "pure topological head" fast path In a narrow but actually quick common case, all topological heads are all on the same branch and all open. In this case, computing the branch map is very simple. We can quickly detect situation where this situation will not change. So we update the V3 format to be able to express this situation and upgrade the update code to detect we remains in that mode. The branch cache is populated with the actual value when the branch map is accessed, but the update_disk method can do the update without needing to populate it.

  $ hg init a
  $ cd a
  $ touch a; hg commit -qAm_
  $ hg bookmark $(for i in $($TESTDIR/seq.py 0 20); do echo b$i; done)
  $ hg clone . ../b -q
  $ cd ../b

Checking that when lookup multiple bookmarks in one go, if one of them
fails (thus causing the sshpeer to be stopped), the errors from the
further lookups don't result in tracebacks.

  $ hg pull -r b0 -r nosuchbookmark $(for i in $($TESTDIR/seq.py 1 20); do echo -r b$i; done) ssh://user@dummy/$(pwd)/../a
  pulling from ssh://user@dummy/$TESTTMP/b/../a
  abort: unknown revision 'nosuchbookmark'
  [255]