odb: fix unnecessary call to `find_cached_object()`

odb: fix unnecessary call to `find_cached_object()`

The function `odb_pretend_object()` writes an object into the in-memory
object database source. The effect of this is that the object will now
become readable, but it won't ever be persisted to disk.

Before storing the object, we first verify whether the object already
exists. This is done by calling `odb_has_object()` to check all sources,
followed by `find_cached_object()` to check whether we have already
stored the object in our in-memory source.

This is unnecessary though, as `odb_has_object()` already checks the
in-memory source transitively via:

  - `odb_has_object()`
  - `odb_read_object_info_extended()`
  - `do_oid_object_info_extended()`
  - `find_cached_object()`

Drop the explicit call to `find_cached_object()`.

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