Add an index for searching projects and their target_refs.
Add an index for searching projects and their target_refs.

Context:
I recently landed https://codereview.chromium.org/797793003/
Prior to that CL Chromium and V8 CLs were uploaded with the target_ref
'refs/heads/master' instead of 'refs/pending/heads/master'.
I need find all Chromium and V8 CLs with 'refs/heads/master' and update them.

BUG=435702
TBR=
NOTRY=true

Review URL: https://codereview.chromium.org/794303003

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra