clone: cut down on global variables in clone.c

clone: cut down on global variables in clone.c

In clone.c the `struct option` which is used to parse the input options
for git-clone(1) is a global variable. Due to this, many variables that
are used to parse the value into, are also global.

Make `builtin_clone_options` a local variable in cmd_clone() and carry
along all variables that are only used in that function.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>