Don't regexp out the short-revid in Git._vcs_commit()

Don't regexp out the short-revid in Git._vcs_commit()

The output version strings change:
  Version 1.5.4.3:
    Created initial commit 217efa7: MESSAGE
    Created commit acb3066: MESSAGE
  Version 1.6.4.4:
    [master (root-commit) c5b48cf] MESSAGE
    [master 66a48c1] MESSAGE
Instead, get the full revid, and look for its beginning in the output.