Add _ostree_repo_open|commit_untrusted_content_bare

Also renames OstreeRepoTrustedContentBareCommit to
OstreeRepoContentBareCommit so that it can be used by both.

This will be needed when we introduce checksum verification of objects
in static deltas.

src/libostree/ostree-repo-static-delta-processing.c

@@ -55,7 +55,7 @@ typedef struct {
   GError        **async_error;
 
   OstreeObjectType output_objtype;
-  OstreeRepoTrustedContentBareCommit barecommitstate;
+  OstreeRepoContentBareCommit barecommitstate;
   guint64          content_size;
   GOutputStream   *content_out;
   char             checksum[65];