gitserver: Empty creatordate leads to bad date format error

Created by: varungandhi-src

We're seeing an error in prod for torvalds/Linux: (logs)

gitserver.RefDescriptions: unexpected output from git for-each-ref (bad date format) \"c39ae07f393806ccf406ef966e9a15afc43cc36arefs/tags/v2.6.11 \"

It seems like the problem is that somehow, one is ending up an empty value for %(creatordate), which the code doesn't handle. It's not super clear when why that is happening here; tags seem to act as roots for git gc, so resetting HEAD, rebasing etc. don't end up affecting the creatordate. Maybe there's some other git operation that leaves tags behind pointing to a non-existent commit?

One unusual thing is that c39ae07f393806ccf406ef966e9a15afc43cc36a isn't a valid commit hash for torvalds/linux right now. However, the GitHub UI does show v2.6.11 as a valid tag. However, if you click on that tag, it gives a 404.