Fix gitc-init behavior

With gitc-init, a gitc client may be specified using '-c'. If we're
not currently in that client, we need to change directories so that
we don't affect the local checkout, and to ensure that repo is
checked out in the new client.

This also makes '-c' optional if already in a gitc client, to match
the rest of the init options.

Change-Id: Ib514ad9fd101698060ae89bb035499800897e9bd