tests: add test for rebasing merges with ancestors of the rebase destination

This shows sub-optimal behaviour. The user gets a merge prompt that it is very
hard to justify.

tests/test-rebase-newancestor.t

@@ -1,55 +1,171 b''
   $ cat >> $HGRCPATH <<EOF
   > [extensions]
   > rebase=
   > 
   > [alias]
   > tglog = log -G --template "{rev}: '{desc}' {branches}\n"
   > EOF
 
   $ hg init repo
   $ cd repo
 
   $ echo A > a
   $ echo >> a
   $ hg ci -Am A
   adding a
 
   $ echo B > a
   $ echo >> a
   $ hg ci -m B
 
   $ echo C > a
   $ echo >> a
   $ hg ci -m C
 
   $ hg up -q -C 0
 
   $ echo D >> a
   $ hg ci -Am AD
   created new head
 
   $ hg tglog
   @  3: 'AD'
   |
   | o  2: 'C'
   | |
   | o  1: 'B'
   |/
   o  0: 'A'
   
   $ hg rebase -s 1 -d 3
   merging a
   merging a
   saved backup bundle to $TESTTMP/repo/.hg/strip-backup/*-backup.hg (glob)
 
   $ hg tglog
   o  3: 'C'
   |
   o  2: 'B'
   |
   @  1: 'AD'
   |
   o  0: 'A'
   
 
   $ cd ..
+
+
+Test rebasing of merges with ancestors of the rebase destination - a situation
+that often happens when trying to recover from repeated merging with a mainline
+branch.
+
+The test case creates a dev branch that contains a couple of merges from the
+default branch. When rebasing to the default branch, these merges would be
+merges with ancestors on the same branch. The merges _could_ contain some
+interesting conflict resolutions or additional changes in the merge commit, but
+that is mixed up with the actual merge stuff and there is in general no way to
+separate them.
+
+Note: The dev branch contains _no_ changes to f-default. It might be unclear
+how rebasing of ancestor merges should be handled, but the current behavior
+with spurious prompts for conflicts in files that didn't change seems very
+wrong.
+
+  $ hg init ancestor-merge
+  $ cd ancestor-merge
+
+  $ touch f-default
+  $ hg ci -Aqm 'default: create f-default'
+
+  $ hg branch -q dev
+  $ hg ci -qm 'dev: create branch'
+
+  $ echo stuff > f-dev
+  $ hg ci -Aqm 'dev: f-dev stuff'
+
+  $ hg up -q default
+  $ echo stuff > f-default
+  $ hg ci -m 'default: f-default stuff'
+
+  $ hg up -q dev
+  $ hg merge -q default
+  $ hg ci -m 'dev: merge default'
+
+  $ hg up -q default
+  $ hg rm f-default
+  $ hg ci -m 'default: remove f-default'
+
+  $ hg up -q dev
+  $ hg merge -q default
+  $ hg ci -m 'dev: merge default'
+
+  $ hg up -q default
+  $ echo stuff > f-other
+  $ hg ci -Aqm 'default: f-other stuff'
+
+  $ hg tglog
+  @  7: 'default: f-other stuff'
+  |
+  | o  6: 'dev: merge default' dev
+  |/|
+  o |  5: 'default: remove f-default'
+  | |
+  | o  4: 'dev: merge default' dev
+  |/|
+  o |  3: 'default: f-default stuff'
+  | |
+  | o  2: 'dev: f-dev stuff' dev
+  | |
+  | o  1: 'dev: create branch' dev
+  |/
+  o  0: 'default: create f-default'
+  
+  $ hg clone -qU . ../ancestor-merge-2
+
+Full rebase all the way back from branching point:
+
+  $ hg rebase -r 'only(dev,default)' -d default
+  remote changed f-default which local deleted
+  use (c)hanged version or leave (d)eleted? c
+  local changed f-default which remote deleted
+  use (c)hanged version or (d)elete? c
+  saved backup bundle to $TESTTMP/ancestor-merge/.hg/strip-backup/1d1a643d390e-backup.hg (glob)
+  $ hg tglog
+  o  5: 'dev: merge default'
+  |
+  o  4: 'dev: f-dev stuff'
+  |
+  @  3: 'default: f-other stuff'
+  |
+  o  2: 'default: remove f-default'
+  |
+  o  1: 'default: f-default stuff'
+  |
+  o  0: 'default: create f-default'
+  
+Grafty cherry picking rebasing:
+
+  $ cd ../ancestor-merge-2
+
+  $ hg phase -fdr0:
+  $ hg rebase -r 'children(only(dev,default))' -d default
+  remote changed f-default which local deleted
+  use (c)hanged version or leave (d)eleted? c
+  local changed f-default which remote deleted
+  use (c)hanged version or (d)elete? c
+  saved backup bundle to $TESTTMP/ancestor-merge-2/.hg/strip-backup/ec2c14fb2984-backup.hg (glob)
+  $ hg tglog
+  o  6: 'dev: merge default'
+  |
+  o  5: 'dev: f-dev stuff'
+  |
+  o  4: 'default: f-other stuff'
+  |
+  o  3: 'default: remove f-default'
+  |
+  o  2: 'default: f-default stuff'
+  |
+  | o  1: 'dev: create branch' dev
+  |/
+  o  0: 'default: create f-default'
+