deltas: Handle untrusted checksums faster and more robustly

When reworking the ostree core [to use O_TMPFILE](https://github.com/ostreedev/ostree/pull/369),
I hit an issue in the way the untrusted delta codepath ends up trying
to re-open the file to checksum it.  That's not possible with
`O_TMPFILE` since the fd (which we opened `O_WRONLY`) is the only
accessible reference to the content.

Fix this by changing the delta processing code to update a checksum as
we're doing writes, which is also faster, and ends up simplifying the
code as well.

What would be an even larger simplification here is if we e.g. used a
separate thread calling `write_object()` or something like that; the
main issue I see there is somehow bridging the fact that function
wants a `GInputStream*` but the delta code is generating stream of
writes.

Closes: #392
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -265,14 +265,14 @@ typedef struct {
 } OstreeRepoContentBareCommit;
 
 gboolean
-_ostree_repo_open_trusted_content_bare (OstreeRepo          *self,
-                                        const char          *checksum,
-                                        guint64              content_len,
-                                        OstreeRepoContentBareCommit *out_state,
-                                        GOutputStream      **out_stream,
-                                        gboolean            *out_have_object,
-                                        GCancellable        *cancellable,
-                                        GError             **error);
+_ostree_repo_open_content_bare (OstreeRepo          *self,
+                                const char          *checksum,
+                                guint64              content_len,
+                                OstreeRepoContentBareCommit *out_state,
+                                GOutputStream      **out_stream,
+                                gboolean            *out_have_object,
+                                GCancellable        *cancellable,
+                                GError             **error);
 
 gboolean
 _ostree_repo_commit_trusted_content_bare (OstreeRepo          *self,