ls-files: support --recurse-submodules --stage

ls-files: support --recurse-submodules --stage

e77aa336f1 ("ls-files: optionally recurse into submodules", 2016-10-10)
taught ls-files the --recurse-submodules argument, but only in a limited
set of circumstances. In particular, --stage was unsupported, perhaps
because there was no repo_find_unique_abbrev(), which was only
introduced in 8bb95572b0 ("sha1-name.c: add
repo_find_unique_abbrev_r()", 2019-04-16). This function is needed for
using --recurse-submodules with --stage.

Now that we have repo_find_unique_abbrev(), teach support for this
combination of arguments.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>