Comparing fails for non-hash image tags

[samb1729]

Looks like we're assuming that the image tag is always going to be a valid git revision:

quay.io/weaveworks/scope

GitError (GitProcessError "git log --first-parent '--format=%h::%cd::%an::%s' '--date=iso' 1.5.0..286e4817" 128 "" "fatal: ambiguous argument '1.5.0..286e4817': unknown revision or path not in the working tree.\nUse '--' to separate paths from revisions, like this:\n'git <command> [<revision>...] -- [<file>...]'\n" (Just "/data/compare-revisions/repositories/repos/a0572b1b3b09141f2048f6b0f30469ad332bda633732f17876f82592dca3fe7b-scope.git"))