pager: add tests

We've never had tests for pager, and I want to start experimenting
with ways to clean up the implementation and make it a bit more
graceful.

tests/test-pager.t

@@ -0,0 +1,153 @@
+  $ cat >> fakepager.py <<EOF
+  > import sys
+  > for line in sys.stdin:
+  >     sys.stdout.write('paged! %r\n' % line)
+  > EOF
+
+Enable ui.formatted because pager won't fire without it, and set up
+pager and tell it to use our fake pager that lets us see when the
+pager was running.
+  $ cat >> $HGRCPATH <<EOF
+  > [ui]
+  > formatted = yes
+  > [extensions]
+  > pager=
+  > [pager]
+  > pager = python $TESTTMP/fakepager.py
+  > EOF
+
+  $ hg init repo
+  $ cd repo
+  $ echo a >> a
+  $ hg add a
+  $ hg ci -m 'add a'
+  $ for x in `python $TESTDIR/seq.py 1 10`; do
+  >   echo a $x >> a
+  >   hg ci -m "modify a $x"
+  > done
+
+By default diff and log are paged, but summary is not:
+
+  $ hg diff -c 2 --pager=yes
+  paged! 'diff -r f4be7687d414 -r bce265549556 a\n'
+  paged! '--- a/a\tThu Jan 01 00:00:00 1970 +0000\n'
+  paged! '+++ b/a\tThu Jan 01 00:00:00 1970 +0000\n'
+  paged! '@@ -1,2 +1,3 @@\n'
+  paged! ' a\n'
+  paged! ' a 1\n'
+  paged! '+a 2\n'
+
+  $ hg log --limit 2
+  paged! 'changeset:   10:46106edeeb38\n'
+  paged! 'tag:         tip\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 10\n'
+  paged! '\n'
+  paged! 'changeset:   9:6dd8ea7dd621\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 9\n'
+  paged! '\n'
+
+  $ hg summary
+  parent: 10:46106edeeb38 tip
+   modify a 10
+  branch: default
+  commit: (clean)
+  update: (current)
+  phases: 11 draft
+
+We can enable the pager on summary:
+
+  $ hg --config pager.attend-summary=yes summary
+  paged! 'parent: 10:46106edeeb38 tip\n'
+  paged! ' modify a 10\n'
+  paged! 'branch: default\n'
+  paged! 'commit: (clean)\n'
+  paged! 'update: (current)\n'
+  paged! 'phases: 11 draft\n'
+
+If we completely change the attend list that's respected:
+
+  $ hg --config pager.attend-diff=no diff -c 2
+  diff -r f4be7687d414 -r bce265549556 a
+  --- a/a	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/a	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,2 +1,3 @@
+   a
+   a 1
+  +a 2
+
+  $ hg --config pager.attend=summary diff -c 2
+  diff -r f4be7687d414 -r bce265549556 a
+  --- a/a	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/a	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,2 +1,3 @@
+   a
+   a 1
+  +a 2
+
+If 'log' is in attend, then 'history' should also be paged:
+  $ hg history --limit 2 --config pager.attend=log
+  paged! 'changeset:   10:46106edeeb38\n'
+  paged! 'tag:         tip\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 10\n'
+  paged! '\n'
+  paged! 'changeset:   9:6dd8ea7dd621\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 9\n'
+  paged! '\n'
+
+Possible bug: history is explicitly ignored in pager config, but
+because log is in the attend list it still gets pager treatment.
+
+  $ hg history --limit 2 --config pager.attend=log \
+  >   --config pager.ignore=history
+  paged! 'changeset:   10:46106edeeb38\n'
+  paged! 'tag:         tip\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 10\n'
+  paged! '\n'
+  paged! 'changeset:   9:6dd8ea7dd621\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 9\n'
+  paged! '\n'
+
+Possible bug: history is explicitly marked as attend-history=no, but
+it doesn't fail to get paged because log is still in the attend list.
+
+  $ hg history --limit 2 --config pager.attend-history=no
+  paged! 'changeset:   10:46106edeeb38\n'
+  paged! 'tag:         tip\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 10\n'
+  paged! '\n'
+  paged! 'changeset:   9:6dd8ea7dd621\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 9\n'
+  paged! '\n'
+
+Possible bug: disabling pager for log but enabling it for history
+doesn't result in history being paged.
+
+  $ hg history --limit 2 --config pager.attend-log=no \
+  > --config pager.attend-history=yes
+  changeset:   10:46106edeeb38
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     modify a 10
+  
+  changeset:   9:6dd8ea7dd621
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     modify a 9
+