Ignore "error" from a commit with changes.

Running `git show --quiet ...` on a commit that includes changes (i.e. not a merge commit) gives an exit code of 1. This makes asv think there has been error when in reality everything is fine.

Perhaps it would be worth making a more specific change so that only 1 is ignored? E.g. a 128 exit code from not being in a git repo still counts as an error.