tests/test-single-head-obsolescence-named-branch-A5.t

rust-utils: move the `filesystem_now` function to a util

This is going to be useful for an upcoming `hg update` fastpath.

=========================================
Testing single head enforcement: Case A-5
=========================================

A repository is set to only accept a single head per name (typically named
branch). However, obsolete changesets can make this enforcement more
complicated, because they can be kept visible by other changeset on other
branch.

This case is part of a series of tests checking this behavior.

Category A: Involving obsolescence
TestCase 5: Obsoleting a merge reveals two heads

.. old-state:
..
.. * 3 changesets on branch default (2 on their own branch + 1 merge)
.. * 1 changeset on branch Z (children of the merge)
..
.. new-state:
..
.. * 2 changesets on branch default (merge is obsolete) each a head
.. * 1 changeset on branch Z keeping the merge visible
..
.. expected-result:
..
.. * 2 heads detected (because we skip the merge)
..
.. graph-summary:
..
..   C ●      (branch Z)
..     |
..   M ⊗
..     |\
..   A ● ● B
..     |/
..     ●

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

  $ cat >> $HGRCPATH << EOF
  > [command-templates]
  > log = "{node|short} [{branch}] ({phase}): {desc}\n"
  > EOF

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

  $ mkdir A5
  $ cd A5
  $ setuprepos single-head
  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
  $ hg merge
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg ci -m 'M0'
  $ hg branch Z
  marked working directory as branch Z
  (branches are permanent and global, did you want a bookmark?)
  $ mkcommit C0
  $ hg push --new-branch
  pushing to $TESTTMP/A5/server
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 3 changesets with 2 changes to 2 files
  $ hg debugobsolete `getid "desc(M0)"` --record-parents
  1 new obsolescence markers
  obsoleted 1 changesets
  1 new orphan changesets
  $ hg heads
  61c95483cc12 [Z] (draft): C0
  74ff5441d343 [default] (draft): B0
  8aaa48160adc [default] (draft): A0
  $ hg log -G --hidden
  @  61c95483cc12 [Z] (draft): C0
  |
  x    14d3d4d41d1a [default] (draft): M0
  |\
  | o  74ff5441d343 [default] (draft): B0
  | |
  o |  8aaa48160adc [default] (draft): A0
  |/
  o  1e4be0697311 [default] (public): root
  

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

(force push to make sure we get the changeset on the remote)

  $ hg push -r 'desc("C0")' --force
  pushing to $TESTTMP/A5/server
  searching for changes
  no changes found
  transaction abort!
  rollback completed
  abort: rejecting multiple heads on branch "default"
  (2 heads: 8aaa48160adc 74ff5441d343)
  [255]