Fix DiffusionMercurialHistoryQuery for file history

Summary: When querying history of a path, we should continue past branchpoints. See D5146 for more discussion.

Test Plan:
Viewing history of a file on a branch which never modified the file no longer fatals.

(Arguably we could render something like "this file was never modified on this branch" and maybe link to the branch where the branchpoint stems from, but that seems of limited use.)

Reviewers: DurhamGoode, vrana, chad

Reviewed By: DurhamGoode

CC: aran

Differential Revision: https://secure.phabricator.com/D5148