Use the new `associated_tags` table to determine which tags are
associated with which. Iterate the associations between tags in a
tree-like manner and do this at the moment the `delete` command is
received. Break up the `associated_tags` links at this time, and
determine which tags would be left dangling and mark all of these for
deletion. If no tags are left after this process, mark the entire call
for deletion.
The previous approach was cumbersome and prone to errors. Using tag
names and branch names to determine which tags are associated with which
is a pointless hurdle, and using a table of associations that is
explicitly kept for this purpose is a much cleaner approach. Also
postponing the decision about which tags to delete until the time the
deletion actually happens can lead to tags not being deleted, when they
really should be (e.g. A -> B, delete A, A -> C).
Change-Id: I03ae57d0a2117ecd721372c1a49468fc34dd630c
Richard Fuchs
53dbef7e1a