Merge branch 'jk/is-promisor-object-keep-tree-in-use' into maint

An earlier optimization discarded a tree-object buffer that is
still in use, which has been corrected.

* jk/is-promisor-object-keep-tree-in-use:
  is_promisor_object(): fix use-after-free of tree buffer

diff --cc packfile.c
Simple merge

diff --cc t/t5616-partial-clone.sh
Simple merge