clone: abort if no dir name could be guessed

clone: abort if no dir name could be guessed

Due to various components of the URI being stripped off it may
happen that we fail to guess a directory name. We currently error
out with a message that it is impossible to create the working
tree '' in such cases. Instead, error out early with a sensible
error message hinting that a directory name should be specified
manually on the command line.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>