tests/test-empty-file.t

tests: duplicate test for pager for old extension and for in-core pager

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 65a3b4d67a65 (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.

  $ hg init a
  $ cd a
  $ touch empty1
  $ hg add empty1
  $ hg commit -m 'add empty1'

  $ touch empty2
  $ hg add empty2
  $ hg commit -m 'add empty2'

  $ hg up -C 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ touch empty3
  $ hg add empty3
  $ hg commit -m 'add empty3'
  created new head

  $ hg heads
  changeset:   2:a1cb177e0d44
  tag:         tip
  parent:      0:1e1d9c4e5b64
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     add empty3
  
  changeset:   1:097d2b0e17f6
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     add empty2
  

  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

Before changeset 05257fd28591, we didn't notice the
empty file that came from rev 1:

  $ hg status
  M empty2
  $ hg commit -m merge
  $ hg manifest --debug tip
  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty1
  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty2
  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty3

  $ cd ..