t7006: add tests for how git branch paginates

t7006: add tests for how git branch paginates

The next couple of commits will change how `git branch` handles
`pager.branch`, 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`.

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 some
tests for `--list` (implied), one for `--edit-description`, and one for
`--set-upstream-to` as a representative of "something other than the
first two".

In particular, use `test_expect_failure` to document that we currently
respect the pager-configuration with `--edit-description`. 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).

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