repo: Add ostree_repo_write_regfile

This API is push rather than pull, which makes it much more
suitable to use cases like parsing a tar file from external
code.

Now, we have a large mess in this area internally because
the original file writing code was pull based, but static
deltas hit the same problem of wanting a push API, so I added
this special `OstreeRepoBareContent` just for writing regular
files from a push API.

Eventually...I'd like to deprecate the pull based API,
and rework things so that for regular files the push API
is the default, and then `write_content_object()` would
be split up into archive/bare cases.

In this world the `ostree_repo_write_content()` API would
then need to hackily bridge pull to push and it'd be
less efficient.

Anyways for now due to this bifurcation, this API only
works on non-archive repositories, but that's fine for
now because that's what I want for the `ostree-ext-container`
bits.

src/libostree/ostree-repo-commit.c

@@ -2855,6 +2855,39 @@ ostree_repo_write_symlink (OstreeRepo       *self,
   return ostree_checksum_from_bytes (csum);
 }
 
+/**
+ * ostree_repo_write_regfile:
+ * @self: Repo,
+ * @expected_checksum: (allow-none): Expected checksum (SHA-256 hex string)
+ * @uid: user id
+ * @gid: group id
+ * @mode: Unix file mode
+ * @content_len: Expected content length
+ * @xattrs: (allow-none): Extended attributes (GVariant type `(ayay)`)
+ * @error: Error
+ * 
+ * Create an `OstreeContentWriter` that allows streaming output into
+ * the repository.
+ *
+ * Returns: (transfer full): A new writer, or %NULL on error
+ * Since: 2021.2
+ */
+OstreeContentWriter *    
+ostree_repo_write_regfile (OstreeRepo       *self,
+                           const char       *expected_checksum,
+                           guint32           uid,
+                           guint32           gid,
+                           guint32           mode,
+                           guint64           content_len,
+                           GVariant         *xattrs,
+                           GError          **error)
+{
+  if (self->mode == OSTREE_REPO_MODE_ARCHIVE)
+    return glnx_null_throw (error, "Cannot currently use ostree_repo_write_regfile() on an archive mode repository");
+
+  return _ostree_content_writer_new (self, expected_checksum, uid, gid, mode, content_len, xattrs, error);
+}
+
 typedef struct {
   OstreeRepo *repo;
   char *expected_checksum;