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
0b75ef5168