test-rebase-conflicts: add a test case about turning obsstore on and off

Turning obsstore and allowunstable on, rebase will skip the
"can't remove original changesets with unrebased descendants" check. Then
rebase could be interrupted (merge conflict), and the user has a chance to
turn off obsstore. If rebase continues, the current code may strip
irrelevant commits (in the test case added, "C" got stripped unexpectedly).

The test case reproduces issue5606. It will be fixed by the "multidest"
rebase refactoring being reviewed. The test case itself is relatively
separate from the rebase refactoring, therefore sent separately hoping to
reduce the number of patches of the main rebase series.

tests/test-rebase-conflicts.t

@@ -3,6 +3,7 b''
   > usegeneraldelta=yes
   > [extensions]
   > rebase=
+  > drawdag=$TESTDIR/drawdag.py
   > 
   > [phases]
   > publish=False
@@ -363,3 +364,50 b' Test minimization of merge conflicts'
   +b
   +c
   +>>>>>>> source: 7bc217434fc1 - test: abc
+
+Test rebase with obsstore turned on and off (issue5606)
+
+  $ cd $TESTTMP
+  $ hg init b
+  $ cd b
+  $ hg debugdrawdag <<'EOS'
+  > D
+  > |
+  > C
+  > |
+  > B E
+  > |/
+  > A
+  > EOS
+
+  $ hg update E -q
+  $ echo 3 > B
+  $ hg commit --amend -m E -A B -q
+  $ hg rebase -r B+D -d . --config experimental.evolution=all
+  rebasing 1:112478962961 "B" (B)
+  merging B
+  warning: conflicts while merging B! (edit, then use 'hg resolve --mark')
+  unresolved conflicts (see hg resolve, then hg rebase --continue)
+  [1]
+
+  $ echo 4 > B
+  $ hg resolve -m
+  (no more unresolved files)
+  continue: hg rebase --continue
+  $ hg rebase --continue --config experimental.evolution=none
+  rebasing 1:112478962961 "B" (B)
+  not rebasing ignored 2:26805aba1e60 "C" (C)
+  rebasing 3:f585351a92f8 "D" (D)
+  saved backup bundle to $TESTTMP/b/.hg/strip-backup/f585351a92f8-cb2a9b47-backup.hg (glob)
+
+  $ rm .hg/localtags
+  $ hg tglog
+  o  3:draft 'D'
+  |
+  o  2:draft 'B'
+  |
+  @  1:draft 'E'
+  |
+  o  0:draft 'A'
+  
+Note the above graph is wrong since C got stripped incorrectly.