lenient hg date parsing

Hah, reminds me of 6885fcfbb4c7b8e0f955a457e2c56ec2907fd2f0. I should've checked for other occurrences.

Interesting that "hg cmd" test passes, it's the "hg native" one that fails. See https://github.com/sourcegraph/go-vcs/pull/13#issuecomment-59159673.