Summary: When running "arc diff" in a mixed-base-revision working copy, we
prevent the operation. Relax this restriction so that having a different root
revision is okay, so long as all affected files share the same revision. This
facilitates multiple similar edits without updates.
Test Plan: Reverted a file to an older revision in an SVN working copy, ran
"arc diff", was rejected. Applied patch, ran "arc diff", diff went through and
was recorded with the right SVN base revision in the database.
Reviewers:
CC: