Allow git revision to refer to revs not available from default clone/fetch

Previously, the "revision" parameter must be something reachable after
doing a default "git clone". Remove this limitation.

A primary use-case is to integrate OBS with the gerrit code review tool,
which stores changes under review in "refs/changes/...". Without this
change, such refs can't be retrieved by the tar_scm service. (forwarded request 122007 from rohanpm)