odb/source: make `read_object_info()` function pluggable

odb/source: make `read_object_info()` function pluggable

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Note that this function is a bit less straight-forward to convert
compared to the other functions. The reason here is that the logic to
read an object is:

  1. We try to read the object. If it exists we return it.

  2. If the object does not exist we reprepare the object database
     source.

  3. We then try reading the object info a second time in case the
     reprepare caused it to appear.

The second read is only supposed to happen for the packfile store
though, as reading loose objects is not impacted by repreparing the
object database.

Ideally, we'd just move this whole logic into the ODB source. But that's
not easily possible because we try to avoid the reprepare unless really
required, which is after we have found out that no other ODB source
contains the object, either. So the logic spans across multiple ODB
sources, and consequently we cannot move it into an individual source.

Instead, introduce a new flag `OBJECT_INFO_SECOND_READ` that tells the
backend that we already tried to look up the object once, and that this
time around the ODB source should try to find any new objects that may
have surfaced due to an on-disk change.

With this flag, the "files" backend can trivially skip trying to re-read
the object as a loose object. Furthermore, as we know that we only try
the second read via the packfile store, we can skip repreparing loose
objects and only reprepare the packfile store.

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