Merge branch 'js/coverity-fixes-null-safety'

Various code paths have been hardened against potential NULL-pointer
dereferences and invalid file descriptor accesses flagged by
Coverity.

* js/coverity-fixes-null-safety:
  shallow: give write_one_shallow() its own hex buffer
  shallow: fix NULL dereference
  bisect: ensure non-NULL `head` before using it
  pack-bitmap: handle missing bitmap for base MIDX
  revision: avoid dereferencing NULL in `add_parents_only()`
  replay: die when --onto does not peel to a commit
  bisect: handle NULL commit in `bisect_successful()`
  mailsplit: move NULL check before first use of file handle
  reftable/stack: guard against NULL list_file in stack_destroy
  remote: guard `remote_tracking()` against NULL remote
  diff: handle NULL return from repo_get_commit_tree()
  diffcore-break: guard against NULLed queue entries in merge loop

diff --cc pack-bitmap.c
Simple merge

diff --cc reftable/stack.c
Simple merge

diff --cc replay.c
Simple merge

diff --cc revision.c
Simple merge

diff --cc shallow.c
Simple merge

diff --cc t/t0410-partial-clone.sh
Simple merge