R1:112346ee6190

Validate inline comment changeset IDs and revision IDs

Summary:
Historically, we had a bug at some point which caused inline comments to get
associated with changeset 0. Prevent that explicitly. See T108.

Test Plan:
Set "$changeset = 0" in the endpoint and got an exception.

Reviewed By: aran
Reviewers: aran
CC: aran
Differential Revision: 374