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>
diff --git a/builtin/difftool.c b/builtin/difftool.c
index ee8dce0..df7e75f 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -20,6 +20,7 @@
 #include "argv-array.h"
 #include "strbuf.h"
 #include "lockfile.h"
+#include "object-store.h"
 #include "dir.h"
 
 static char *diff_gui_tool;