Let ls-files strip trailing slashes in submodules' paths

Let ls-files strip trailing slashes in submodules' paths

Tab completion makes it easy to add a trailing slash to a submodule path.
As it is completely clear what the user actually wanted to say, be nice
and strip that slash at the end.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>