t7006: add tests for how git config paginates

t7006: add tests for how git config paginates

The next couple of commits will change how `git config` handles
`pager.config`, similar to how de121ffe5 (tag: respect `pager.tag` in
list-mode only, 2017-08-02) and ff1e72483 (tag: change default of
`pager.tag` to "on", 2017-08-02) changed `git tag`. Similar work has
also been done to `git branch`.

Add tests in this area to make sure that we don't regress and so that
the upcoming commits can be made clearer by adapting the tests. Add
tests for simple config-setting, `--edit`, `--get`, `--get-urlmatch`,
`get-all`, and `--list`. Those represent a fair portion of the various
options that will be affected by the next two commits.

Use `test_expect_failure` to document that we currently respect the
pager-configuration with `--edit`. The current behavior is buggy since
the pager interferes with the editor and makes the end result completely
broken. See also b3ee740c8 (t7006: add tests for how git tag paginates,
2017-08-02).

The next commit will teach simple config-setting and `--get` to ignore
`pager.config`. Test the current behavior as "success", not "failure",
since the currently expected behavior according to documentation would
be to page. The next commit will change that expectation by updating the
documentation on `git config` and will redefine those successful tests.

Remove the test added in commit 3ba7e6e29a (config: run
setup_git_directory_gently() sooner, 2010-08-05) since it has some
overlap with these. We could leave it or tweak it, or place new tests
like these next to it, but let's instead make the tests for `git config`
as similar as possible to the ones for `git tag` and `git branch`, and
place them after those.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>