Code review comment for lp:~spiv/bzr/checkout-tags-propagation-603395-2.2

I should point out: this does have the perhaps unwanted side-effect that "bzr merge" can cause tags to be added the master even if the next command is "bzr commit". That's really <https://bugs.launchpad.net/bzr/+bug/99137> (tags are "permanently" propagated by merge), and this patch is just making the overall behaviour more consistent. I can see an argument that undoing the accidental limiting of the the effect of bug 99137 might not be acceptable in a stable branch, though.

A compromise would be to add a new optional argument to merge_to (or perhaps a whole new method) that callers can use to explicitly enable/disable propagation. I'm not sure if it would be better to default to enabled (so that plugins like builddeb can automatically benefit), or disabled (to minimise the change in behaviour, and also perhaps to avoid needing to tweak the API layers between cmd_merge and br.tags.merge_to to pass the flag). With 'bzr merge' not propagating we'd then rely on <https://code.launchpad.net/~spiv/bzr/tags-commit-propagation-603395-2.2> to propagate at commit time rather than merge time.