The memory usage discipline of tag_op_list_t is never to free the
internal array of tag operations before freeing the whole list, so it
makes sense to take advantage of hierarchical de-allocation by talloc.
By not relying on the context passed into tag_parse_line, we can allow
tag_op_list_t structures to live longer than that context.
The idea is not to bother with restore operations if they don't change
the set of tags. This is actually a relatively common case.
In order to avoid fancy datastructures, this method is quadratic in
the number of tags; at least on my mail database this doesn't seem to
be a big problem.
These are meant to be shared between notmuch-tag and notmuch-restore.
The bulk of the routines implement a "tag operation list" abstract
data type act as a structured representation of a set of tag
operations (typically coming from a single tag command or line of
input).
