R1:66af361f10d8

Fix a Mercurial issue where split heads would be detected incorrectly

Summary: Ref T5197. When searching for split branch heads, we incorrectly consider descendant heads of other branches. This can cause us to detect a split tip when one does not exist (the old tip is the branch tip, but other descendant heads exist). Instead, consider only heads on the same branch.

Test Plan:
Repro is something like this:

- `hg update default`
- `hg branch branch1; hg commit ...`
- `hg push`
- `hg update…