obsolescence: add test for the "branch replacement" logic during push, case B6

Mercurial checks for the introduction of new heads on push. Evolution comes
into play to detect if existing branches on the server are being replaced by
some of the new one we push.

This changeset adds test for the improved "branch replacement" logic introduce
in an earlier commits. This tests initially lived in the evolve extensions.
Since we now have the code handling this logic in core, it make sense to have
the tests in core too.

See inline documentation for details about the test case added in this
changeset.

tests/test-push-checkheads-pruned-B6.t

@@ -0,0 +1,78 b''
+====================================
+Testing head checking code: Case B-6
+====================================
+
+Mercurial checks for the introduction of new heads on push. Evolution comes
+into play to detect if existing branches on the server are being replaced by
+some of the new one we push.
+
+This case is part of a series of tests checking this behavior.
+
+Category B: simple case involving pruned changesets
+TestCase 6: single changesets, pruned then superseeded (on a new changeset)
+
+.. old-state:
+..
+.. * 1 changeset branch
+..
+.. new-state:
+..
+.. * old branch is rewritten onto another one,
+.. * the new version is then pruned.
+..
+.. expected-result:
+..
+.. * push allowed
+..
+.. graph-summary:
+..
+..   A ø⇠⊗ A'
+..     | |
+..     | ◔ B
+..     |/
+..     ●
+
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+Test setup
+----------
+
+  $ mkdir B6
+  $ cd B6
+  $ setuprepos
+  creating basic server and client repo
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd client
+  $ hg up 0
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ mkcommit B0
+  created new head
+  $ mkcommit A1
+  $ hg up 'desc(B0)'
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg debugobsolete `getid "desc(A0)"` `getid "desc(A1)"`
+  $ hg debugobsolete --record-parents `getid "desc(A1)"`
+  $ hg log -G --hidden
+  x  ba93660aff8d (draft): A1
+  |
+  @  74ff5441d343 (draft): B0
+  |
+  | x  8aaa48160adc (draft): A0
+  |/
+  o  1e4be0697311 (public): root
+  
+
+Actual testing
+--------------
+
+  $ hg push
+  pushing to $TESTTMP/B6/server (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  2 new obsolescence markers
+
+  $ cd ../..