reset: strip trailing slash on input path

When using tab-completion, a directory path will often end with a
trailing slash which currently confuses "git rm" when dealing with
submodules.  Now that we have parse_pathspec we can easily handle this
by simply adding the PATHSPEC_STRIP_SUBMODULE_SLASH_CHEAP flag.

Signed-off-by: John Keeping <john@keeping.me.uk>
rm: re-use parse_pathspec's trailing-slash removal

Instead of re-implementing the "remove trailing slashes" loop in
builtin/rm.c just pass PATHSPEC_STRIP_SUBMODULE_SLASH_CHEAP to
parse_pathspec.

Signed-off-by: John Keeping <john@keeping.me.uk>