teach init-repository about branches

specifically, add the --branch option and the necessary keys to
.gitmodules.

Change-Id: I09e2df6bd012629f76a35238d66697637bdffe44
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Fix description of --mirror argument

The argument actually requires a trailing slash.

Change-Id: Idc2567746d2b3f4ebf31c2bce5ec683d2a1f425b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Fix cloning with mirrors

Commit 486f5f37761 broke the cloning with one of the mirror options:

git ls-remote git://mirror/qt/qtactiveqt.git.git test/if/mirror/exists
fatal: The remote end hung up unexpectedly

Change-Id: Ia154ee34dd10e35b24676777d8dde98ef0790ee1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
don't fetch unless we are updating

it's just wasting time.

Change-Id: I541cc535c5c95b0140cfa14d7cb042dbe029525e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>