odb: remove mutual recursion when parsing alternates

odb: remove mutual recursion when parsing alternates

When adding an alternative object database source we not only have to
consider the added source itself, but we also have to add _its_ sources
to our database. We implement this via mutual recursion:

  1. We first call `link_alt_odb_entries()`.

  2. `link_alt_odb_entries()` calls `parse_alternates()`.

  3. We then add each alternate via `odb_add_alternate_recursively()`.

  4. `odb_add_alternate_recursively()` calls `link_alt_odb_entries()`
     again.

This flow is somewhat hard to follow, but more importantly it means that
parsing of alternates is somewhat tied to the recursive behaviour.

Refactor the function to remove the mutual recursion between adding
sources and parsing alternates. The parsing step thus becomes completely
oblivious to the fact that there is recursive behaviour going on at all.
The recursion is handled by `odb_add_alternate_recursively()` instead,
which now recurses with itself.

This refactoring allows us to move parsing of alternates into object
database sources in a subsequent step.

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