obsolescence: add test for the "branch replacement" logic during push, case D6

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.

The current code for this logic is very basic (eg: issue4354) and was poorly
tested. We have a better implementation coming in the evolve extension fixing
these issues and with more serious tests coverage. In the process of upstreaming
this improved logic, we start with adding the test case that are already passing
with the current implementation. Once they are all in, we'll upstream the better
implementation and the extra test cases.

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

tests/test-push-checkheads-unpushed-D6.t

@@ -0,0 +1,82
+====================================
+Testing head checking code: Case D-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 D: remote head is "obs-affected" locally, but result is not part of the push
+TestCase 6: single changeset, superseeded then pruned (on a new changeset unpushed) changeset
+
+This is a partial push variation of case B-6
+
+.. old-state:
+..
+.. * 1 changeset branch
+..
+.. new-state:
+..
+.. * old branch is rewritten onto another one,
+.. * the new version is then pruned.
+..
+.. expected-result:
+..
+.. * push denied
+..
+.. graph-summary:
+..
+..   A ø⇠⊗ A'
+..     | |
+.. C ◔ | ○ B
+..    \|/
+..     ●
+
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+Test setup
+----------
+
+  $ mkdir D6
+  $ cd D6
+  $ 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 '0'
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ mkcommit C0
+  created new head
+  $ hg debugobsolete `getid "desc(A0)"` `getid "desc(A1)"`
+  $ hg debugobsolete --record-parents `getid "desc(A1)"`
+  $ hg log -G --hidden
+  @  0f88766e02d6 (draft): C0
+  |
+  | x  ba93660aff8d (draft): A1
+  | |
+  | o  74ff5441d343 (draft): B0
+  |/
+  | x  8aaa48160adc (draft): A0
+  |/
+  o  1e4be0697311 (public): root
+  
+
+Actual testing
+--------------
+
+  $ hg push --rev 'desc(C0)'
+  pushing to $TESTTMP/D6/server (glob)
+  searching for changes
+  abort: push creates new remote head 0f88766e02d6!
+  (merge or see 'hg help push' for details about pushing new heads)
+  [255]
+
+  $ cd ../..