This patch series supports for only outputing the "objects" (OID)
with a new option names `--oid-only`.

Changes with the first patch are :

        1. Three commits are squashed to 1 commit (Peff's advice)
        2. The tests issues (Đoàn Trần Công Danh's advice)
        3. Use `OPT_CMDMODE()` for mutually exclusive control
           (Ævar Arnfjörð Bjarmason's advice)

Some discussions are not included in Patch 2 :

        1. `git ls-tree --long --name-only` and
           `git ls-tree --long --oid-only` which is arguably a bug
           (Peff's advice)
        2. Support `--format` for `git-ls-tree`
           (Ævar Arnfjörð Bjarmason's advice)

The reason why these 2 discussions not included is I'm not sure whether
I should continue on the current patchset or start a new one. And for the
second, I think current implementation is clear and simple to use, meeting
the needs of the moment. Maybe I will to support `--format` option, but
before that, I'm appreciate if there are more suggestions appear.

Thanks.

Teng Long (1):
  ls-tree.c: support `--oid-only` option for "git-ls-tree"

 Documentation/git-ls-tree.txt |  8 +++++--
 builtin/ls-tree.c             | 27 ++++++++++++++++-------
 t/t3104-ls-tree-oid.sh        | 40 +++++++++++++++++++++++++++++++++++
 3 files changed, 65 insertions(+), 10 deletions(-)
 create mode 100755 t/t3104-ls-tree-oid.sh

Range-diff against v1:
1:  c4479178d7 < -:  ---------- ls-tree.c: support `--oid-only` option for "git-ls-tree"
2:  853ebbcf88 < -:  ---------- t3104: add related tests for `--oid-only` option
3:  33c68c1f11 < -:  ---------- git-ls-tree.txt: description of the 'oid-only' option
-:  ---------- > 1:  8b68568d6c ls-tree.c: support `--oid-only` option for "git-ls-tree"
-- 
2.33.1.10.g1f74a882e4