object-store: move object access functions to object-store.h

This should make these functions easier to find and cache.h less
overwhelming to read.

In particular, this moves:
- read_object_file
- oid_object_info
- write_object_file

As a result, most of the codebase needs to #include object-store.h.
In this patch the #include is only added to files that would fail to
compile otherwise.  It would be better to #include wherever
identifiers from the header are used.  That can happen later
when we have better tooling for it.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "object-store.h"
 #include "commit.h"
 #include "blob.h"
 #include "diff.h"