prefixcmp(): fix-up mechanical conversion.

Previous step converted use of strncmp() with literal string
mechanically even when the result is only used as a boolean:

    if (!strncmp("foo", arg, 3)) ==> if (!(-prefixcmp(arg, "foo")))

This step manually cleans them up to read:

    if (!prefixcmp(arg, "foo"))

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-grep.c

@@ -527,9 +527,9 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			opt.word_regexp = 1;
 			continue;
 		}
-		if (!(-prefixcmp(arg, "-A")) ||
-		    !(-prefixcmp(arg, "-B")) ||
-		    !(-prefixcmp(arg, "-C")) ||
+		if (!prefixcmp(arg, "-A") ||
+		    !prefixcmp(arg, "-B") ||
+		    !prefixcmp(arg, "-C") ||
 		    (arg[0] == '-' && '1' <= arg[1] && arg[1] <= '9')) {
 			unsigned num;
 			const char *scan;