Fix multi-commit uploading with Mercurial in pre-commit review mode.

Review Request #12623 — Created Sept. 22, 2022 and submitted — Latest diff uploaded


Review Board


Currently, uploading a multi-commit review request diffs fails with
"API Error 207: Repository File Not Found" when different files are
changed in different patches.

See the report here:

It tries to request a raw file from a server repository using the
revision ID of the previous uploaded commit. Sure there's no such
revision on the server as those patches aren't pushed yet to the
upstream repository in a pre-commit review case.

For a proper file request in this case it should use "base_commit_id".

Reproduced the original problem in a new database, and verified that
this fixes the issue.