Use git config to make overlapping remotes in a test repo

Instead of `git remote`.

Since Git 2.51.0, path collisions in remote names are automatically
detected and rejected when `git remote add` is used to attempt to
add them, since creating such collisions is usually unintentional:
git/git@a5a727c

However, we deliberately do this in our test suite, in the
`make_remote_config_repos.sh` fixture script. When Git 2.51.0 came
to be used on CI, some new test failures arose as a result, as
observed in #2130.

This fixes the script by using two `git config` commands rather
than a `git remote` command to produce the overlap. This technique
is (intentionally) still allowed in Git 2.51.0.

Author: EliahKagan

gix/tests/fixtures/make_remote_config_repos.sh

@@ -140,7 +140,8 @@ git clone fetch multiple-remotes
   git remote add with/two/slashes ../fetch
   git fetch with/two/slashes
 
-  git remote add with/two ../fetch
+  git config remote.with/two.url ../fetch
+  git config remote.with/two.fetch '+refs/heads/*:refs/remotes/with/two/*'
   git fetch with/two
 
   # add a specialised refspec mapping