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-archive.c

@@ -35,7 +35,7 @@ static int run_remote_archiver(const char *remote, int argc,
 
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
-		if (!(-prefixcmp(arg, "--exec="))) {
+		if (!prefixcmp(arg, "--exec=")) {
 			if (exec_at)
 				die("multiple --exec specified");
 			exec = arg + 7;