git-clone: Support changing the origin branch with -o

Earlier, git-clone stored upstream's master in the branch named 'origin',
possibly overwriting an existing such branch.

Now you can change it by calling git-clone with '-o <other_name>'.

[jc: added ref format check, subdirectory safety, documentation
 and usage string.]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>