patch, 3 partssupport `--oid-only` in `ls-tree`

Sometimes, we only want to get the objects from output of `ls-tree` and commands like `sed` or `cut` is usually used to intercept the origin output to achieve this purpose in practical.

The patch contains three commits

    1. Implementation of the option.
    2. Add new tests in "t3104".
    3. Documentation modifications.

I'm appreciate if someone help to review the patch.

Thanks.

Teng Long (3):
  ls-tree.c: support `--oid-only` option for "git-ls-tree"
  t3104: add related tests for `--oid-only` option
  git-ls-tree.txt: description of the 'oid-only' option

 Documentation/git-ls-tree.txt |  8 +++--
 builtin/ls-tree.c             | 11 +++++++
 t/t3104-ls-tree-oid.sh        | 55 +++++++++++++++++++++++++++++++++++
 3 files changed, 72 insertions(+), 2 deletions(-)
 create mode 100755 t/t3104-ls-tree-oid.sh