histedit: simplify computation of `newchildren` during --continue

We are now checking for any changesets between the previous `parentctx` and the
current working directory parent. If the current working directory parent is
inconsistent, we abort.

This change is useful as it simplifies the --continue process, easing upcoming
changes.

While working on this changeset, I spotted an unhandled corner case. This corner
case is now documented and have an appropriate issue in the tracker (issue3655).
However, the corner case is still unhandled. handling this test case would
required some additional work:

- actually decide what the proper behavior should be:
- change content of "histedit-state" to add missing data necessary to detect
  the situation
- add proper testcase,

But leaving the case unhandled is "okay":

- this is not a regression,
- this is not the purpose of the current series,
- the freeze was near and I had more critical stuff to attend to,
- this is a simple but non trivial, (see above)

hgext/histedit.py

@@ -550,21 +550,20 b' def histedit(ui, repo, *parent, **opts):'
 
 def bootstrapcontinue(ui, repo, parentctx, existing, replacemap, rules,
                       tmpnodes, created, replaced, opts):
-    currentparent, wantnull = repo.dirstate.parents()
-    # existing is the list of revisions initially considered by
-    # histedit. Here we use it to list new changesets, descendants
-    # of parentctx without an 'existing' changeset in-between. We
-    # also have to exclude 'existing' changesets which were
-    # previously dropped.
-    descendants = set(c.node() for c in
-            repo.set('(%d::) - %d', parentctx, parentctx))
-    notdropped = set(n for n in existing if n in descendants and
-            (n not in replacemap or replacemap[n] in descendants))
-    # Discover any nodes the user has added in the interim. We can
-    # miss changesets which were dropped and recreated the same.
-    newchildren = list(c.node() for c in repo.set(
-        'sort(%ln - (%ln or %ln::))', descendants, existing, notdropped))
     action, currentnode = rules.pop(0)
+    # is there any new commit between the expected parent and "."
+    #
+    # note: does not take non linear new change in account (but previous
+    #       implementation didn't used them anyway (issue3655)
+    newchildren = [c.node() for c in repo.set('(%d::.)', parentctx)]
+    if not newchildren:
+        # `parentctxnode` should match but no result. This means that
+        # currentnode is not a descendant from parentctxnode.
+        msg = _('working directory parent is not a descendant of %s')
+        hint = _('update to %s or descendant and run "hg histedit '
+                 '--continue" again') % parentctx
+        raise util.Abort(msg % parentctx, hint=hint)
+    newchildren.pop(0)  # remove parentctxnode
     if action in ('f', 'fold'):
         tmpnodes.extend(newchildren)
     else:

tests/test-histedit-edit.t

@@ -66,6 +66,19 b' edit the history'
   abort: Make changes as needed, you may commit or record as needed now.
   When you are finished, run hg histedit --continue to resume.
 
+Go at a random point and try to continue
+
+  $ hg id -n
+  3+
+  $ hg up 0
+  0 files updated, 0 files merged, 3 files removed, 0 files unresolved
+  $ HGEDITOR='echo foobaz > ' hg histedit --continue
+  abort: working directory parent is not a descendant of 055a42cdd887
+  (update to 055a42cdd887 or descendant and run "hg histedit --continue" again)
+  [255]
+  $ hg up 3
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
 commit, then edit the revision
   $ hg ci -m 'wat'
   created new head