The API call will fail when Bitbucket repositories are private. In
that case, probe for the repository using vcsCmd.ping.

      Fixes #5375

Change-Id: Ia604ecf9014805579dfda4b5c8e627a52783d56e
Reviewed-on: https://go-review.googlesource.com/1910Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>