lib: Use a common helper function to compare checksums

So we get a consistent error message; came up in a PR review.

Closes: #1277
Approved by: jlebon

src/libostree/ostree-core-private.h

@@ -141,6 +141,12 @@ _ostree_validate_bareuseronly_mode_finfo (GFileInfo  *finfo,
   return _ostree_validate_bareuseronly_mode (content_mode, checksum, error);
 }
 
+gboolean
+_ostree_compare_object_checksum (OstreeObjectType objtype,
+                                 const char      *expected,
+                                 const char      *actual,
+                                 GError         **error);
+
 gboolean
 _ostree_parse_delta_name (const char  *delta_name,
                           char        **out_from,