config: Fix passing a git hash as revision

See discussion at !76 (closed)

When passing a hash as the repo revision, we currently attempt to do a git reset origin/<hash> which fails. The git reset is only meant to sync local branches with the remote, when the repo revision is a branch. Use git show-ref to test if the revision is a branch, and only perform the git reset in this case.

Reported-by: Sudeep Holla sudeep.holla@arm.com Signed-off-by: Jean-Philippe Brucker jean-philippe@linaro.org