ls-files: align format atoms with ls-tree

ls-files: align format atoms with ls-tree

"git ls-files --format" can be used to format the output of
multiple file entries in the index, while "git ls-tree --format"
can be used to format the contents of a tree object. However,
the current set of %(objecttype), "(objectsize)", and
"%(objectsize:padded)" atoms supported by "git ls-files --format"
is a subset of what is available in "git ls-tree --format".

Users sometimes need to establish a unified view between the index
and tree, which can help with comparison or conversion between the two.

Therefore, this patch adds the missing atoms to "git ls-files --format".
"%(objecttype)" can be used to retrieve the object type corresponding
to a file in the index, "(objectsize)" can be used to retrieve the
object size corresponding to a file in the index, and "%(objectsize:padded)"
is the same as "(objectsize)", except with padded format.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>