Don't throw an exception if the author of a tag can't be found

Summary:
It's possible to have a tag with no tagger (or git used to allow this),
so some tags (like 26791a8bcf0e6d33f43aef7682bdb555236d56de in the linux
kernel) were causing trouble.

Test Plan:
opened diffusion to a page where I was previously getting a red box complaining
about being unable to parse the output of git for-each-ref.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, vrana

Differential Revision: https://secure.phabricator.com/D2711