Ensure resetting on commit id

When reset command was called with tag name as parameter the resulting
HEAD was set to the tag's SHA-1 which is a bug. This patch ensures that
repository.resolve() call always returns commit id.

Change-Id: I219b898c620a75c497c8652dbf4735fd094c4d7c
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>