(cvs|svn)import: Ask git-tag to overwrite old tags.

If the tag was moved in CVS or SVN history, it will be moved in the
imported history as well. Tag history is not tracked.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-svnimport.perl

@@ -873,7 +873,7 @@ sub commit {
 
 		$dest =~ tr/_/\./ if $opt_u;
 
-		system('git-tag', $dest, $cid) == 0
+		system('git-tag', '-f', $dest, $cid) == 0
 			or die "Cannot create tag $dest: $!\n";
 
 		print "Created tag '$dest' on '$branch'\n" if $opt_v;