branchmap: simplify update code
We drop iterrevs which are not needed anymore. The know head are never a
descendant of the updated set. It was possible with the old strip code. This
simplification make the code easier to read an update.
$ hg init 1
$ echo '[ui]' >> 1/.hg/hgrc
$ echo 'timeout = 10' >> 1/.hg/hgrc
$ echo foo > 1/foo
$ hg --cwd 1 ci -A -m foo
adding foo
$ hg clone 1 2
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg clone 2 3
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo '[hooks]' >> 2/.hg/hgrc
$ echo 'changegroup.push = hg push -qf ../1' >> 2/.hg/hgrc
$ echo bar >> 3/foo
$ hg --cwd 3 ci -m bar
$ hg --cwd 3 push ../2
pushing to ../2
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files