run_external_diff: drop fflush(NULL)

This fflush was added in d5535ec7 (Use run_command() to spawn
external diff programs instead of fork/exec., 2007-10-19),
because flushing buffers before forking is a good habit.

But later, 7d0b18a4 (Add output flushing before fork(),
2008-08-04) added it to the generic run-command interface,
meaning that our flush here is redundant.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -2932,7 +2932,6 @@ static void run_external_diff(const char *pgm,
 		argv_array_push(&argv, pgm);
 		argv_array_push(&argv, name);
 	}
-	fflush(NULL);
 
 	argv_array_pushf(&env, "GIT_DIFF_PATH_COUNTER=%d", ++o->diff_path_counter);
 	argv_array_pushf(&env, "GIT_DIFF_PATH_TOTAL=%d", q->nr);