Code review comment for lp:~adeuring/launchpad/fix-broken-initialisation-of-bug-latest-patch-uploaded

I added an index for bugattachment.bug.

jtv noticed that we don't have such an index at present. Without it, the inital update query for the column bug.latest_patch_updated takes quite a long time; finding bug attachments for a given bug will benefit as well from this index.