Remove archaic use of regex capture \1 in favour of $1

Using it will generate a perl warning "\1 better written as $1".

Signed-off-by: Nick Woolley <git.wu-lee@noodlefactory.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-cvsexportcommit.perl

@@ -299,7 +299,7 @@ foreach my $f (@files) {
 	while (<FILTER_IN>)
 	{
 	    my $line = $_;
-	    $line =~ s/\$([A-Z][a-z]+):[^\$]+\$/\$\1\$/g;
+	    $line =~ s/\$([A-Z][a-z]+):[^\$]+\$/\$$1\$/g;
 	    print FILTER_OUT $line;
 	}
 	close FILTER_IN;