tests/test-histedit-merge-tools.t

rust: switch hg-core and hg-cpython to rust 2018 edition

Many interesting changes have happened in Rust since the Oxidation Plan was
introduced, like the 2018 edition and procedural macros:

    - Opting in to the 2018 edition is a clear benefit in terms of future
      proofing, new (nice to have) syntactical sugar notwithstanding. It
      also has a new non-lexical, non-AST based borrow checker that has
      fewer bugs(!) and allows us to write correct code that in some cases
      would have been rejected by the old one.
    - Procedural macros allow us to use the PyO3 crate which maintainers have
      expressed the clear goal of compiling on stable, which would help in
      code maintainability compared to rust-cpython.

In this patch are the following changes:
    - Removing most `extern crate` uses
    - Updating `use` clauses (`crate` keyword, nested `use`)
    - Removing `mod.rs` in favor of an aptly named module file

Like discussed in the mailing list (
https://www.mercurial-scm.org/pipermail/mercurial-devel/2019-July/132316.html
), until Rust integration in Mercurial is considered to be out of the
experimental phase, the maximum version of Rust allowed is whatever the latest
version Debian packages.

Differential Revision: https://phab.mercurial-scm.org/D6597

Test histedit extension: Merge tools
====================================

Initialization
---------------

  $ . "$TESTDIR/histedit-helpers.sh"

  $ cat >> $HGRCPATH <<EOF
  > [alias]
  > logt = log --template '{rev}:{node|short} {desc|firstline}\n'
  > [extensions]
  > histedit=
  > mockmakedate = $TESTDIR/mockmakedate.py
  > [ui]
  > pre-merge-tool-output-template='pre-merge message for {node}\n'
  > EOF

Merge conflict
--------------

  $ hg init r
  $ cd r
  $ echo foo > file
  $ hg add file
  $ hg ci -m "First" -d "1 0"
  $ echo bar > file
  $ hg ci -m "Second" -d "2 0"

  $ hg logt --graph
  @  1:2aa920f62fb9 Second
  |
  o  0:7181f42b8fca First
  

Invert the order of the commits, but fail the merge.
  $ hg histedit --config ui.merge=false --commands - 2>&1 <<EOF | fixbundle
  > pick 2aa920f62fb9 Second
  > pick 7181f42b8fca First
  > EOF
  merging file
  pre-merge message for b90fa2e91a6d11013945a5f684be45b84a8ca6ec
  merging file failed!
  Fix up the change (pick 7181f42b8fca)
  (hg histedit --continue to resume)

  $ hg histedit --abort | fixbundle
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

Invert the order of the commits, and pretend the merge succeeded.
  $ hg histedit --config ui.merge=true --commands - 2>&1 <<EOF | fixbundle
  > pick 2aa920f62fb9 Second
  > pick 7181f42b8fca First
  > EOF
  merging file
  pre-merge message for b90fa2e91a6d11013945a5f684be45b84a8ca6ec
  7181f42b8fca: skipping changeset (no changes)