src/ostree: Add --commit-only option to ostree prune

Recently we have noticed exceedingly long execution times
for multiple invocations of ostree prune. This is a result of
calculating full reachability on each invocation.

The --commit-only flag provides an alternative strategy. It will only
traverse and delete commit objects to avoid the more expensive
reachability calculations. This allows us to chain multiple --commit-only
commands cheaply, and then follow with a more expensive ostree prune
invocation at the end to clean up orphaned meta and content objects.

src/libostree/libostree-devel.sym

@@ -20,6 +20,11 @@
    - uncomment the include in Makefile-libostree.am
 */
 
+LIBOSTREE_2022.2 {
+global:
+  ostree_repo_traverse_commit_with_flags;
+} LIBOSTREE_2021.5;
+
 /* Stub section for the stable release *after* this development one; don't
  * edit this other than to update the year.  This is just a copy/paste
  * source.  Replace $LASTSTABLE with the last stable version, and $NEWVERSION