bisect: use the standard 'if (!var)' way to check for 0

Instead of using 'var == 0' in an if condition, let's use '!var' and
make 'bisect.c' more consistent with the rest of the code.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -572,7 +572,7 @@ static int sqrti(int val)
 {
 	float d, x = val;
 
-	if (val == 0)
+	if (!val)
 		return 0;
 
 	do {
@@ -869,7 +869,7 @@ static void check_good_are_ancestors_of_bad(struct repository *r,
 		goto done;
 
 	/* Bisecting with no good rev is ok. */
-	if (good_revs.nr == 0)
+	if (!good_revs.nr)
 		goto done;
 
 	/* Check if all good revs are ancestor of the bad rev. */