Add an archive-z repository mode

This is where loose content objects are stored as one compressed file,
instead of the two separate ones for regular archive mode.  This mode
would be suitable for HTTP servers, beause only one HTTP request is
necessary, and the result would be compressed.

src/libostree/ostree-repo.h

@@ -44,7 +44,8 @@ GFile *       ostree_repo_get_path (OstreeRepo  *self);
 
 typedef enum {
   OSTREE_REPO_MODE_BARE,
-  OSTREE_REPO_MODE_ARCHIVE
+  OSTREE_REPO_MODE_ARCHIVE,
+  OSTREE_REPO_MODE_ARCHIVE_Z
 } OstreeRepoMode;
 
 gboolean       ostree_repo_mode_from_string (const char      *mode,