fetch known hashes
fetch known hashes

When running as a bot (even locally if you want), grab the known hashes
from Gold and scan through FM's stdout looking for unknown hashes.

If we do find unknown hashes, requeue the batch for individual reruns
like we do on failure, print the command and new hash if those singleton
reruns also (re)produce an unknown hash.

Eventually, I'll have singleton runs write out .pngs and upload them to
Gold when the hash is unknown.

Change-Id: I835881e6e6260e4dbe84de8d03d16921881aae1c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/363039
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia