phases: add `hg help phases` hint to failures to edit public commits

There were a couple of locations that were missing this hint after an
edition of some kind failed due to the public phase.

hgext/histedit.py

@@ -928,7 +928,8 b' def between(repo, old, new, keep):'
             raise util.Abort(_('cannot edit history that contains merges'))
         root = ctxs[0] # list is already sorted by repo.set
         if not root.mutable():
-            raise util.Abort(_('cannot edit public changeset: %s') % root)
+            raise util.Abort(_('cannot edit public changeset: %s') % root,
+                             hint=_('see "hg help phases" for details'))
     return [c.node() for c in ctxs]
 
 def makedesc(repo, action, rev):

mercurial/obsolete.py

@@ -1215,7 +1215,8 b' def createmarkers(repo, relations, flag='
 
             if not prec.mutable():
                 raise util.Abort("cannot obsolete public changeset: %s"
-                                 % prec)
+                                 % prec,
+                                 hint='see "hg help phases" for details')
             nprec = prec.node()
             nsucs = tuple(s.node() for s in sucs)
             npare = None

tests/test-histedit-obsolete.t

@@ -220,6 +220,7 b' Check that histedit respect immutability'
   
   $ hg histedit -r '.~2'
   abort: cannot edit public changeset: cb9a9f314b8b
+  (see "hg help phases" for details)
   [255]