repo-file: s/content_checksum/contents_checksum/

This is what we call it everywhere else, so just be consistent.
It also lines up with metadata_checksum better.

https://bugzilla.gnome.org/show_bug.cgi?id=707727

src/libostree/ostree-repo-file.h

@@ -65,9 +65,7 @@ void ostree_repo_file_tree_set_metadata (OstreeRepoFile *self,
                                           const char     *checksum,
                                           GVariant       *metadata);
 
-void ostree_repo_file_tree_set_content_checksum (OstreeRepoFile  *self,
-                                                  const char      *checksum);
-const char *ostree_repo_file_tree_get_content_checksum (OstreeRepoFile  *self);
+const char *ostree_repo_file_tree_get_contents_checksum (OstreeRepoFile  *self);
 
 gboolean ostree_repo_file_is_tree (OstreeRepoFile  *self);