This patch makes us respect pager.attend again if the extension is
enabled. It also brings back the default attend list, so e.g. summary
is not paged by default, just like it used to be before pager was
moved into core.
When the old pager extension is enabled, I think we should try to be
as BC as reasonable. To help with that, this patch brings back
test-pager.t as of 45ff5bf9e9c2 (pager: add a test of --pager=no
functionality, 2017-02-06), but under the name test-pager-legacy.t
However, since the behavior has changed in a few cases (notably by no
longer respecting pager.attend), the file is modified to work with the
current version. We will recover some lost BC in coming patches.
Also, to make sure the in-core pager does not depend on the pager
extension being enabled, this patch disables the extension in
test-pager.t. It turns out that pager.attend-$cmd was only supported
when the pager extension was enabled, so the tests are updated to
reflect that. We will need to decide what to do with these.
