ls-files: optionally recurse into submodules

ls-files: optionally recurse into submodules

Allow ls-files to recognize submodules in order to retrieve a list of
files from a repository's submodules.  This is done by forking off a
process to recursively call ls-files on all submodules. Use top-level
--super-prefix option to pass a path to the submodule which it can
use to prepend to output or pathspec matching logic.

Signed-off-by: Brandon Williams <redacted>
Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>