This function (and the helpers `ancestry` and `find_and_peel`) are
eagerly peeling to commits, leading to updates of tags that target the
same commit to be missed.
For example, there could be tag two tag objects with *different* OIDs
A and B, from *different* authors, using *different* tag names, signed
with *different* secret keys, and both pointing to *the same* commit C.
The implementation would consider A and B to be the same, just becuase
A and B both peel to C, skipping the update.
This is counterintuitive, and when combined with canonical references
can be quite confusing.
Change this to only reason about an ancestry if the two objects in
question really both are commits directly. Otherwise, treat cases where
no structure can be used as ancestry similarly to non-fast-forward
updates.
9c8ab7fa62