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

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>