reset: handle submodule with trailing slash

When using tab-completion, a directory path will often end with a
trailing slash which currently confuses "git reset" 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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jk/trailing-slash-in-pathspec' into pu

* jk/trailing-slash-in-pathspec:
  rm: re-use parse_pathspec's trailing-slash removal
  reset: handle submodule with trailing slash