Re: [PATCH v4 1/6] t1300: make tests more robust with non-default ref backends

>
> The t1300 test suite exercises the git-config(1) tool. To do so, the
> test overwrites ".git/config" to contain custom contents in several
> places with code like the following:
>
> ```
> cat > .git/config <<\EOF
> ...
> EOF
> ```

> While this is easy enough to do, it may create problems when using a
> non-default repository format because this causes us to overwrite the
> repository format version as well as any potential extensions. With the
> upcoming "reftable" ref backend the result is that Git would try to
> access refs via the "files" backend even though the repository has been
> initialized with the "reftable" backend, which will cause failures when
> trying to access any refs.
>
> Ideally, we would rewrite the whole test suite to not depend on state
> written by previous tests, but that would result in a lot of changes in
> this test suite.

> Instead, we only refactor tests which access the refdb
> to be more robust by using their own separate repositories, which allows
> us to be more careful and not discard required extensions.
>
> Note that we also have to touch up how the CUSTOM_CONFIG_FILE gets
> accessed. This environment variable contains the relative path to a
> custom config file which we're setting up. But because we are now using
> subrepositories, this relative path will not be found anymore because
> our working directory changes. This issue is addressed by storing the
> absolute path to the file in CUSTOM_CONFIG_FILE instead.