Replace all die("BUG: ...") calls by BUG() ones

In d8193743e08 (usage.c: add BUG() function, 2017-05-12), a new macro
was introduced to use for reporting bugs instead of die(). It was then
subsequently used to convert one single caller in 588a538ae55
(setup_git_env: convert die("BUG") to BUG(), 2017-05-12).

The cover letter of the patch series containing this patch
(cf 20170513032414.mfrwabt4hovujde2@sigill.intra.peff.net) is not
terribly clear why only one call site was converted, or what the plan
is for other, similar calls to die() to report bugs.

Let's just convert all remaining ones in one fell swoop.

This trick was performed by this invocation:

	sed -i 's/die("BUG: /BUG("/g' $(git grep -l 'die("BUG' \*.c)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/zlib.c b/zlib.c
index 4223f1a..d594cba 100644
--- a/zlib.c
+++ b/zlib.c
@@ -52,9 +52,9 @@ static void zlib_post_call(git_zstream *s)
 	bytes_consumed = s->z.next_in - s->next_in;
 	bytes_produced = s->z.next_out - s->next_out;
 	if (s->z.total_out != s->total_out + bytes_produced)
-		die("BUG: total_out mismatch");
+		BUG("total_out mismatch");
 	if (s->z.total_in != s->total_in + bytes_consumed)
-		die("BUG: total_in mismatch");
+		BUG("total_in mismatch");
 
 	s->total_out = s->z.total_out;
 	s->total_in = s->z.total_in;