refs: generalize `refs_for_each_fullref_in_prefixes()`

refs: generalize `refs_for_each_fullref_in_prefixes()`

The function `refs_for_each_fullref_in_prefixes()` can be used to
iterate over all references part of any of the user-provided prefixes.
In contrast to the `prefix` parameter of `refs_for_each_ref_ext()` it
knows to handle the case well where multiple of the passed-in prefixes
start with a common prefix by computing longest common prefixes and then
iterating over those.

While we could move this logic into `refs_for_each_ref_ext()`, this one
feels somewhat special as we perform multiple iterations. But what we
_can_ do is to generalize how this function works: instead of accepting
only a small handful of parameters, we can have it accept the full
options structure.

One obvious exception is that the caller must not provide a prefix via
the options. But this case can be easily detected.

Refactor the code accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>