From: ZheNing Hu <adlternative@gmail.com>

`git ls-files --stage` default output format is:

[<tag> ]<mode> <object> <stage> <file>

sometime we want to find a path's corresponding objectname,
we will parse the output and extract objectname from it
again and again.

So introduce a new option `--only-object-name` which can only
output objectname when giving `--stage` or `--resolve-undo`.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1250.git.1654509678718.gitgitgadget@gmail.com