Fix fetch refspecs when not cloning all branches

Fix fetch refspecs when not cloning all branches

When not all branches are cloned, the fetch refspec for the
remote should not be "+refs/heads/*:refs/remotes/origin/*":
that would fetch all branches on the very next fetch, thus
making a clone with only a subset of the branches rather
pointless.

Instead, produce refspecs for the cloned branches only.

Canonical git also does this for its --single-branch case;
it doesn't have an option to clone only a subset of the branches
(only one or all).

Bug: 466858
Change-Id: Ie871880f757663437efac1e8b3313094f9e629b3
Also-by: Julian Enoch <julian.enoch@ericsson.com>
Signed-off-by: Julian Enoch <julian.enoch@ericsson.com>
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>