tests/test-rebase-brute-force.t

dirstate-tree: Add tree traversal/iteration

Like Python’s, Rust’s iterators are "external" in that they are driven
by a caller who calls a `next` method. This is as opposed to "internal"
iterators who drive themselves and call a callback for each item.

Writing an internal iterator traversing a tree is easy with recursion,
but internal iterators cannot rely on the call stack in that way,
they must save in an explicit object all state that they need to be
preserved across two `next` calls.

This algorithm uses a `Vec` as a stack that contains what would be
local variables on the call stack if we could use recursion.

Differential Revision: https://phab.mercurial-scm.org/D10370

  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > drawdag=$TESTDIR/drawdag.py
  > bruterebase=$TESTDIR/bruterebase.py
  > [experimental]
  > evolution.createmarkers=True
  > evolution.allowunstable=True
  > EOF
  $ init() {
  >   N=`expr ${N:-0} + 1`
  >   cd $TESTTMP && hg init repo$N && cd repo$N
  >   hg debugdrawdag
  > }

Source looks like "N"

  $ init <<'EOS'
  > C D
  > |\|
  > A B Z
  > EOS

  $ hg debugbruterebase 'all()-Z' Z
     A: A':Z
     B: B':Z
    AB: A':Z B':Z
     C: ABORT: cannot rebase 3:a35c07e8a2a4 without moving at least one of its parents
    AC: A':Z C':A'B
    BC: B':Z C':B'A
   ABC: A':Z B':Z C':A'B'
     D: D':Z
    AD: A':Z D':Z
    BD: B':Z D':B'
   ABD: A':Z B':Z D':B'
    CD: ABORT: cannot rebase 3:a35c07e8a2a4 without moving at least one of its parents
   ACD: A':Z C':A'B D':Z
   BCD: B':Z C':B'A D':B'
  ABCD: A':Z B':Z C':A'B' D':B'

Moving backwards

  $ init <<'EOS'
  > C
  > |\
  > A B
  > |
  > Z
  > EOS
  $ hg debugbruterebase 'all()-Z' Z
    B: B':Z
    A: 
   BA: B':Z
    C: ABORT: cannot rebase 3:b8d7149b562b without moving at least one of its parents
   BC: B':Z C':B'A
   AC: 
  BAC: B':Z C':B'A