diff: restrict pathspec limitations to diff b/f case only

builtin_diff_b_f() needs a path, not pathspec. Other modes in diff
can deal with pathspec just fine. But because of the current
GUARD_PATHSPEC() location, other modes also reject :(glob) and
:(icase).

Move GUARD_PATHSPEC(), and the "path" assignment statement, which is
the reason of this GUARD_PATHSPEC(), inside builtin_diff_b_f().

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>