lib/repo: Add a new private API for bare content writes

This lowers into the commit core what the static delta code
was doing, and improves the API.

The bigger picture issue is that for writing large files, our current "pull" API
where the caller provides a `GInputStream` is very awkward in some scenarios.
For example, we have a whole "libarchive input stream" that is a ~200 line
GObject that boils down to wrapping `archive_read_data()`.

This came more to a head when I was working on rpm-ostree jigdo since I had to
copy that object.

One step we can take after this is to further split `write_content_object()`
into a "write symlink or archive object" versus "write bare content object"
(it already has a mess of conditionals) and teach the latter case to call
this.

The eventual goal here is to make this API public.

Closes: #1355
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -365,24 +365,41 @@ _ostree_repo_commit_tmpf_final (OstreeRepo        *self,
                                 GCancellable      *cancellable,
                                 GError           **error);
 
+typedef struct {
+  gboolean initialized;
+  gpointer opaque0[10];
+  guint opaque1[10];
+} OstreeRepoBareContent;
+void _ostree_repo_bare_content_cleanup (OstreeRepoBareContent *regwrite);
+G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(OstreeRepoBareContent, _ostree_repo_bare_content_cleanup)
+
 gboolean
-_ostree_repo_open_content_bare (OstreeRepo          *self,
-                                const char          *checksum,
-                                guint64              content_len,
-                                GLnxTmpfile         *out_tmpf,
+_ostree_repo_bare_content_open (OstreeRepo            *self,
+                                const char            *checksum,
+                                guint64                content_len,
+                                guint                  uid,
+                                guint                  gid,
+                                guint                  mode,
+                                GVariant              *xattrs,
+                                OstreeRepoBareContent *out_regwrite,
                                 GCancellable        *cancellable,
                                 GError             **error);
 
 gboolean
-_ostree_repo_commit_trusted_content_bare (OstreeRepo          *self,
-                                          const char          *checksum,
-                                          GLnxTmpfile         *tmpf,
-                                          guint32              uid,
-                                          guint32              gid,
-                                          guint32              mode,
-                                          GVariant            *xattrs,
-                                          GCancellable        *cancellable,
-                                          GError             **error);
+_ostree_repo_bare_content_write (OstreeRepo                 *repo,
+                                 OstreeRepoBareContent      *barewrite,
+                                 const guint8               *buf,
+                                 size_t                      len,
+                                 GCancellable               *cancellable,
+                                 GError                    **error);
+
+gboolean
+_ostree_repo_bare_content_commit (OstreeRepo                 *self,
+                                  OstreeRepoBareContent      *barewrite,
+                                  char                       *checksum_buf,
+                                  size_t                      buflen,
+                                  GCancellable               *cancellable,
+                                  GError                    **error);
 
 gboolean
 _ostree_repo_load_file_bare (OstreeRepo         *self,