clone: fix creation of explicitly named target directory

clone: fix creation of explicitly named target directory

'git clone <repo> path/' (note the trailing slash) fails, because the
entire path is interpreted as leading directories. So when mkdir tries to
create the actual path, it already exists.

This makes sure trailing slashes are removed.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>