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 default; hg commit ...
• hg push - Previously, we would find the head of branch1 and incorrectly account for it as a head of default.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5197

Differential Revision: https://secure.phabricator.com/D9308