t7400: fix broken "submodule add/reconfigure --force" test

This test has been dysfunctional since it was added by 619acfc78c (submodule add: extend force flag to add existing repos, 2016-10-06), however, two problems early in the test went unnoticed due to a broken &&-chain later in the test. First, it tries configuring the submodule with repository "bogus-url", however, "git submodule add" insists that the repository be either an absolute URL or a relative pathname requiring prefix "./" or "../" (this is true even with --force), but "bogus-url" does not meet those criteria, thus the command fails. Second, it then tries configuring a submodule with a path which is .gitignore'd, which is disallowed. This restriction can be overridden with --force, but the test neglects to use that option. Fix both problems, as well as the broken &&-chain behind which they hid. Reviewed-by: Stefan Beller <sbeller@google.com> Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>