Do not call 'cmp' with non-existant -q flag.

I cannot find when that flag was removed if it ever existed, I can find
nothing about it in the ChangeLog and NEWS file of GNU diff.  The current
flag is -s aka --quiet aka --silent, so let's use -s, assuming it is a
portable flag.  Feel free to lart me with a POSIX bible if needed.

Signed-off-by: Yann Dirson <ydirson@altern.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-cvsexportcommit.perl

@@ -151,7 +151,7 @@
     my $blob = `git-ls-tree $tree "$f" | cut -f 1 | cut -d ' ' -f 3`;
     chomp $blob;
     `git-cat-file blob $blob > $tmpdir/blob`;
-    if (system('cmp', '-q', $f, "$tmpdir/blob")) {
+    if (system('cmp', '-s', $f, "$tmpdir/blob")) {
 	warn "Binary file $f in CVS does not match parent.\n";
 	$dirty = 1;
 	next;