branchcache: allow to detect "pure topological case" for branchmap
We don't rum this detection every time we run the branchcache, that would be
costly. However we now do it when running `hg debugupdatecache`.
This will help existing repository to benefit from the fastpath when possible.
#require unix-permissions no-root
$ hg init a
$ cd a
$ echo foo > b
$ hg add b
$ hg ci -m "b"
$ chmod -w .hg/store
$ cd ..
$ hg clone a b
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets 97310831fa1a
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ chmod +w a/.hg/store # let test clean up
$ cd b
$ hg verify -q
$ cd ..