bisect--helper: use '-res' in 'cmd_bisect__helper' return

Following 'enum bisect_error' vocabulary, return variable 'res' is
always non-positive.
Let's use '-res' instead of 'abs(res)' to make the code clearer.

Mentored-by: Christian Couder <[email protected]>
Helped-by: Junio C Hamano <[email protected]>
Signed-off-by: Miriam Rubio <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: mirucam

builtin/bisect--helper.c

@@ -731,5 +731,5 @@ int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 	if (res == BISECT_INTERNAL_SUCCESS_MERGE_BASE)
 		res = BISECT_OK;
 
-	return abs(res);
+	return -res;
 }