Summary:
When falling back to convert_revision to get the svnrev for a commit to return
it as a globalrev, it seems like it would make sense to verify that the
convert_revision is indeed a svnrev (and not, say, a git commit hash from
hggit).
Reviewed By: farnz
Differential Revision: D20891268
fbshipit-source-id: 2451ad787fcce7b10b6a405f2855313ca51f5b3e
