RFR: Improve health check for git repositories

[Robin Westberg via github.com]

Hi all,

Please review this update to the isHealthy function. If a repository contains refs, pick one of those for testing branch creation, since it is possible that HEAD is not available.

Best regards,
Robin

----------------

Commits:
 - 53c3143c:	Even if there are refs, HEAD might not be present

Pull request:
https://git.openjdk.java.net/skara/pull/72

Webrev:
https://webrevs.openjdk.java.net/skara/72/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/72.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/72/head:pull/72