forge: ChangeIsMerged -> ChangesAreMerged

[abhinav]

Replace ChangeIsMerged with a bulk ChangesAreMerged API that takes a list of ChangeIDs and returns a list of booleans in the same order.

With this in place, we can change 'repo sync' to make a single request for all submitted branches instead of one request per branch.

[skip changelog]: No user facing changes in this branch.

[abhinav]

This change is part of the following stack:

• 399 ◀

  • 400

_{Change managed by git-spice.}