sha1_array: let callbacks interrupt iteration

The callbacks for iterating a sha1_array must have a void
return.  This is unlike our usual for_each semantics, where
a callback may interrupt iteration and have its value
propagated. Let's switch it to the usual form, which will
enable its use in more places (e.g., where we are replacing
an existing iteration with a different data structure).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-sha1-array.c

@@ -1,9 +1,10 @@
 #include "cache.h"
 #include "sha1-array.h"
 
-static void print_sha1(const unsigned char sha1[20], void *data)
+static int print_sha1(const unsigned char sha1[20], void *data)
 {
 	puts(sha1_to_hex(sha1));
+	return 0;
 }
 
 int cmd_main(int argc, const char **argv)