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.

Test null revisions (node 0000000000000000000000000000000000000000, aka rev -1)
in various circumstances.

Make an empty repo:

  $ hg init a
  $ cd a

  $ hg files -r 0000000000000000000000000000000000000000
  [1]
  $ hg files -r .
  [1]

Add an empty commit (this makes the changelog refer to a null manifest node):


  $ hg commit -m "init" --config ui.allowemptycommit=true

  $ hg files -r .
  [1]

Strip that empty commit (this makes the changelog file empty, as opposed to missing):

  $ hg --config 'extensions.strip=' strip . > /dev/null

  $ hg files -r .
  [1]