R1:4dc857e36d54

Fix an issue with incorrect split head detection in Mercurial after pushing a medley of varied changes

Summary:
Fixes T10665. See that task for discussion.

Because `$head_map` is not properly re-initialized for each ref we check, pushes which affect multiple branches (say, "A" and "B") can have information bleed from the first branch check to the second branch.

To trigger a problem behavior, you can push one commit which updates an existing branch, plus one commit which creates a new branch. If they…