pathspec: use BUG(...) not die("BUG:%s:%d....", <file>, <line>)

pathspec: use BUG(...) not die("BUG:%s:%d....", <file>, <line>)

Change code that was added in 8f4f8f4579f (guard against new pathspec
magic in pathspec matching code, 2013-07-14) to use the BUG() macro
instead of emitting a "fatal" message with the "__FILE__"-name and
"__LINE__"-numbers.

The original code predated the existence of the BUG() function, which
was added in d8193743e08 (usage.c: add BUG() function, 2017-05-12).

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>