branchcache: cleanup the final key generation after update
A lot of duplicated work seemed to be done, as we already update the tiprev and
tipnode when needed right before. So we simplify that part to focus on the
filtered hash.
See inline comment for details.
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]