Old behaviour of inheriting tags with values was that tags added to, for
instance, a posting, would be added to the tags, possibly overriding the
tags of the transaction. Or, in other words, the transaction tags were
added to the posting tags, as there was no sense of overriding a tag.

The new behaviour is that tags are now overridden when a lower level
re-uses that tag name. For instance, when defining a transaction with
tag t:v and posting with tag t:v2, only t:v2 remains on the posting,
overriding the transaction tag.

Perhaps it would be desirable to add an option for additive tags over
overriding tags, as it may well be useful at times to add to tags in the
parent.