object-file: get rid of `the_repository` in `read_loose_object()`

object-file: get rid of `the_repository` in `read_loose_object()`

The function `read_loose_object()` takes a path to an object file and
tries to parse it. As such, the function does not depend on any specific
object database but instead acts as an ODB-independent way to read a
specific file. As such, all it needs as input is a repository so that we
can derive repo settings and the hash algorithm.

That repository isn't passed in as a parameter though, as we implicitly
depend on the global `the_repository`. Refactor the function so that we
pass in the repository as a parameter.

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