histedit: suggest "histedit --abort" for inconsistent histedit state

Mercurial earlier than 2.7 allows users to do anything other than
starting new histedit, even though current histedit is not finished or
aborted yet. So, unfinished (and maybe inconsistent now) histedit
states may be left and forgotten in repositories.

Before this patch, histedit extension shows the message below, when it
detects such inconsistent state:

   abort: REV is not an ancestor of working directory
   (update to REV or descendant and run "hg histedit --continue" again)

But this message is incorrect, unless old Mercurial is re-installed,
because Mercurial 2.7 or later disallows users to update the working
directory to another revision.

This patch changes the hint message to suggest "hg histedit --abort".

hgext/histedit.py

@@ -646,8 +646,7 b' def bootstrapcontinue(ui, repo, parentct'
             # `parentctxnode` should match but no result. This means that
             # currentnode is not a descendant from parentctxnode.
             msg = _('%s is not an ancestor of working directory')
-            hint = _('update to %s or descendant and run "hg histedit '
-                     '--continue" again') % parentctx
+            hint = _('use "histedit --abort" to clear broken state')
             raise util.Abort(msg % parentctx, hint=hint)
         newchildren.pop(0)  # remove parentctxnode
     # Commit dirty working directory if necessary

tests/test-histedit-arguments.t

@@ -70,6 +70,35 b' Run on a revision not ancestors of the c'
   [255]
   $ hg up --quiet
 
+Run on a revision not descendants of the initial parent
+--------------------------------------------------------------------
+
+Test the message shown for inconsistent histedit state, which may be
+created (and forgotten) by Mercurial earlier than 2.7. This emulates
+Mercurial earlier than 2.7 by renaming ".hg/histedit-state"
+temporarily.
+
+  $ HGEDITOR=cat hg histedit -r 4 --commands - << EOF
+  > edit 08d98a8350f3 4 five
+  > EOF
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  reverting alpha
+  Make changes as needed, you may commit or record as needed now.
+  When you are finished, run hg histedit --continue to resume.
+  [1]
+
+  $ mv .hg/histedit-state .hg/histedit-state.back
+  $ hg update --quiet --clean 2
+  $ mv .hg/histedit-state.back .hg/histedit-state
+
+  $ hg histedit --continue
+  abort: c8e68270e35a is not an ancestor of working directory
+  (use "histedit --abort" to clear broken state)
+  [255]
+
+  $ hg histedit --abort
+  $ hg update --quiet --clean
+
 Test that missing revisions are detected
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