revision: rename bit to `do_not_die_on_missing_objects`
The bit `do_not_die_on_missing_tree` is used in revision.h to ensure the
revision walker does not die when encountering a missing tree. This is
currently exclusively set within `builtin/rev-list.c` to ensure the
`--missing` option works with missing trees.
In the upcoming commits, we will extend `--missing` to also support
missing commits. So let's rename the bit to
`do_not_die_on_missing_objects`, which is object type agnostic and can
be used for both trees/commits.
Signed-off-by: Karthik Nayak <karthik.188@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>