repo: Allocate a tmpdir for each OstreeFetcher to isolate concurrent downloads

This way two pulls will not use the same tmpdir and accidentally
overwrite each other. However, consecutive OstreeFetchers will reuse
the tmpdirs, so that we can properly resume downloading large objects.

https://bugzilla.gnome.org/show_bug.cgi?id=757611

src/libostree/ostree-fetcher.h

@@ -54,8 +54,10 @@ typedef enum {
 
 GType   _ostree_fetcher_get_type (void) G_GNUC_CONST;
 
-OstreeFetcher *_ostree_fetcher_new (int                     tmpdir_dfd,
-                                    OstreeFetcherConfigFlags   flags);
+OstreeFetcher *_ostree_fetcher_new (int                      tmpdir_dfd,
+                                    OstreeFetcherConfigFlags flags,
+                                    GCancellable            *cancellable,
+                                    GError                 **error);
 
 int  _ostree_fetcher_get_dfd (OstreeFetcher *fetcher);