From: ZheNing Hu <adlternative@gmail.com>

Add a new option "--format" that outputs index entries
informations in a custom format, taking inspiration
from the option with the same name in the `git ls-tree`
command.

"--format" cannot used with "-s", "-o", "-k", "-t",
" --resolve-undo","--deduplicate" and "--eol".

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

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