Add 'git svn help [cmd]' which works outside a repo.

Previously there was no explicit 'help' command, but 'git svn help'
still printed the usage message (as an invalid command), provided you
got past the initialization steps that required a valid repo.
Signed-off-by: NBen Jackson <ben@ben.com>
Acked-by: NEric Wong <normalperson@yhbt.net>

git-svn.perl

@@ -219,6 +219,9 @@ BEGIN
 		$cmd = $ARGV[$i];
 		splice @ARGV, $i, 1;
 		last;
+	} elsif ($ARGV[$i] eq 'help') {
+		$cmd = $ARGV[$i+1];
+		usage(0);
 	}
 };