odb/source: generalize `reprepare()` callback

The `reprepare()` callback function can be used to flush caches of a
given object source and then prepare it anew. This is for example used
when a concurrent process may have written new objects. Ultimately, this
can be seen as doing two separate steps:

  1. We drop any caches.

  2. We prepare the source.

We have one callsite in git-grep(1) though that really only want to do
(2). This is done by reaching into the "files" backend directly and then
calling `odb_source_packed_prepare()`, which of course may not work with
alternate backends.

We could in theory just call `reprepare()` here, and that would likely
not have any significant downside. But this would certainly feel like a
code smell.

Instead, generalize the `reprepare()` callback to `prepare()` with a
flag that optionally instructs the backend to also flush the caches,
which allows us to drop the external `odb_source_packed_prepare()`
declaration.

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

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