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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
