tests/test-push-checkheads-multibranches-E2.t

documentation: add some internals documentation about bid merge

This is an important algorithm that was only documented on the wiki so far.

Some update to the algorithm (and associated doc) is to expected in the future
since the bid merge algorithm is bug-ridden when it comes to file deletion comes
to play.

Differential Revision: https://phab.mercurial-scm.org/D8711

====================================
Testing head checking code: Case E-2
====================================

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 E: case involving changeset on multiple branch
TestCase 8: moving interleaved branch away from each other

.. old-state:
..
.. * 2-changeset on branch default
.. * 1-changeset on branch Z (between the two other)
..
.. new-state:
..
.. * 2-changeset on branch default, aligned
.. * 1-changeset on branch Z (at the same location)
..
.. expected-result:
..
.. * push allowed
..
.. graph-summary:
..
..   C ø⇠◔ C'
..     | |
..   B ◔ |
..     | |
..   A ø⇠◔ A'
..     |/
..     ●

  $ . $TESTDIR/testlib/push-checkheads-util.sh

Test setup
----------

  $ mkdir E1
  $ cd E1
  $ 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 branch Z
  marked working directory as branch Z
  (branches are permanent and global, did you want a bookmark?)
  $ mkcommit B0
  $ hg branch default --force
  marked working directory as branch default
  $ mkcommit C0
  created new head
  $ hg push --new-branch
  pushing to $TESTTMP/E1/server
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 2 changesets with 2 changes to 2 files
  $ hg up 0
  0 files updated, 0 files merged, 3 files removed, 0 files unresolved
  $ mkcommit A1
  created new head
  $ mkcommit C1
  $ hg debugobsolete `getid "desc(A0)" ` `getid "desc(A1)"`
  1 new obsolescence markers
  obsoleted 1 changesets
  2 new orphan changesets
  $ hg debugobsolete `getid "desc(C0)" ` `getid "desc(C1)"`
  1 new obsolescence markers
  obsoleted 1 changesets
  $ hg log -G --hidden
  @  0c76bc104656 (draft): C1
  |
  o  f6082bc4ffef (draft): A1
  |
  | x  afc55ba2ce61 (draft): C0
  | |
  | *  93e5c1321ece (draft): B0
  | |
  | x  8aaa48160adc (draft): A0
  |/
  o  1e4be0697311 (public): root
  

Actual testing
--------------

  $ hg push -r 'desc("C1")'
  pushing to $TESTTMP/E1/server
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 2 changesets with 2 changes to 2 files (+1 heads)
  2 new obsolescence markers
  obsoleted 2 changesets
  1 new orphan changesets

  $ cd ../..