tag: use git.update_ref()

tag: use git.update_ref()

Even tag refs can be packed by git, so writing to a file
might cause issues. Call git update-ref for tags as well.

In order to do that properly (and allow 'bup tag' to keep
its ability to modify tags), change git.update_ref() to
take a 'force' argument that allows updating a ref even
if it already exists with an old value we don't know.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>