die("bug"): report bugs consistently

die("bug"): report bugs consistently

The vast majority of error messages in Git's source code which report a
bug use the convention to prefix the message with "BUG:".

As part of cleaning up merge-recursive to stop die()ing except in case of
detected bugs, let's just make the remainder of the bug reports consistent
with the de facto rule.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>