perms batch mode confuses; print something to help

What happens is that running

    ssh git@host perms reponame

appears to hang, since it is waiting for STDIN.  I added a message to
help, since we don't want users losing files accidentally!

(The other alternative is to add a specific option for batch mode, but
this is backward incompatible for people who have scripts that may be
doing this).

thanks to Caleb Cushing for catching this

----

The "make sure Ctrl-C gets caught" thing needs some explanation.

Without it, a user could inadvertently lose his gl-perms file if he ran
the command in batch mode.  You'd think that the Ctrl-C would hit the

    for (<>) {

line and bail, but it manages to reach the

    _print( $pf, @a );

line somehow.  Even trapping SIG INT does not help.

I suspect it is to do with how signals are propagated by ssh across a
"no-pty" session, but am not sure.

src/commands/mirror

@@ -48,7 +48,7 @@ if ( $cmd eq 'push' ) {
     if (-f "gl-creator") {
         # try to propagate the wild repo, including creator name and gl-perms
         my $creator = `cat gl-creator`; chomp($creator);
-        trace(1, `cat gl-perms 2>/dev/null | ssh $host CREATOR=$creator perms -c \\'$repo\\'`);
+        trace(1, `cat gl-perms 2>/dev/null | ssh $host CREATOR=$creator perms -c \\'$repo\\' 2>/dev/null`);
     }
 
     my $errors = 0;

src/commands/perms

@@ -84,12 +84,15 @@ sub setperms {
 
     if ( not @_ ) {
         # legacy mode; pipe data in
+        print STDERR "'batch' mode started, waiting for input (run with '-h' for details).\n";
+        print STDERR "Please hit Ctrl-C if you did not intend to do this.\n";
         @ARGV = ();
         my @a;
         for (<>) {
             _die "Invalid role '$1'; check the rc file" if /(\S+)/ and not $rc{ROLES}{$1};
             push @a, $_;
         }
+        print STDERR "\n";  # make sure Ctrl-C gets caught
         _print( $pf, @a );
         return;
     }