obsolescence: add test for the "branch replacement" logic during push, case D3

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-unpushed-D3.t

@@ -0,0 +1,110 b''
+====================================
+Testing head checking code: Case D-3
+====================================
+
+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 3: multi-changeset branch, split on multiple new others, only one of them is pushed
+
+.. old-state:
+..
+.. * 2 changesets branch
+..
+.. new-state:
+..
+.. * 2 new branches, each superseding one changeset in the old one.
+..
+.. expected-result:
+..
+.. * pushing only one of the resulting branch (either of them)
+.. * push denied
+..
+.. graph-summary:
+..
+.. B'◔⇢ø B
+..   | |
+.. A | ø⇠◔ A'
+..   | |/
+..    \|
+..     ●
+
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+Test setup
+----------
+
+  $ mkdir D3
+  $ cd D3
+  $ setuprepos
+  creating basic server and client repo
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd server
+  $ mkcommit B0
+  $ hg up 0
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ cd ../client
+  $ hg pull
+  pulling from $TESTTMP/D3/server (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  (run 'hg update' to get a working copy)
+  $ hg up 0
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ mkcommit A1
+  created new head
+  $ hg up '0'
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ mkcommit B1
+  created new head
+  $ hg debugobsolete `getid "desc(A0)" ` `getid "desc(A1)"`
+  $ hg debugobsolete `getid "desc(B0)" ` `getid "desc(B1)"`
+  $ hg log -G --hidden
+  @  25c56d33e4c4 (draft): B1
+  |
+  | o  f6082bc4ffef (draft): A1
+  |/
+  | x  d73caddc5533 (draft): B0
+  | |
+  | x  8aaa48160adc (draft): A0
+  |/
+  o  1e4be0697311 (public): root
+  
+
+Actual testing
+--------------
+
+  $ hg push --rev 'desc(A1)'
+  pushing to $TESTTMP/D3/server (glob)
+  searching for changes
+  abort: push creates new remote head f6082bc4ffef!
+  (merge or see 'hg help push' for details about pushing new heads)
+  [255]
+  $ hg push --rev 'desc(B1)'
+  pushing to $TESTTMP/D3/server (glob)
+  searching for changes
+  abort: push creates new remote head 25c56d33e4c4!
+  (merge or see 'hg help push' for details about pushing new heads)
+  [255]
+
+Extra testing
+-------------
+
+In this case, even a bare push is creating more heads
+
+  $ hg push
+  pushing to $TESTTMP/D3/server (glob)
+  searching for changes
+  abort: push creates new remote head 25c56d33e4c4!
+  (merge or see 'hg help push' for details about pushing new heads)
+  [255]
+
+  $ cd ../..