histedit: allow operation from non-head if obsolete is enabled

Obsolescence markers can represent this situation just fine. Rewritten
revisions are marked as precursors of the ones creates by
histedit. Unaffected descendants become "unstable".

If obsolescence is not enabled we keep the current behavior of
aborting. This new behavior only applies when obsolete is
enabled and is subject to future discussion and changes.

hgext/histedit.py

@@ -593,7 +593,8 b' def between(repo, old, new, keep):'
     When keep is false, the specified set can't have children."""
     ctxs = list(repo.set('%n::%n', old, new))
     if ctxs and not keep:
-        if repo.revs('(%ld::) - (%ld + hidden())', ctxs, ctxs):
+        if (not obsolete._enabled and
+            repo.revs('(%ld::) - (%ld + hidden())', ctxs, ctxs)):
             raise util.Abort(_('cannot edit history that would orphan nodes'))
         root = ctxs[0] # list is already sorted by repo.set
         if not root.phase():

tests/test-histedit-obsolete.t

@@ -141,3 +141,15 b' Check that histedit respect phases'
   $ hg histedit -r '.~2'
   abort: cannot edit immutable changeset: cb9a9f314b8b
   [255]
+
+
++Test ui.prevent-unstable option
++------------------------------------
+
+  $ hg up '.^'
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg phase --force --draft .
+  $ hg log -r 'children(.)'
+  9:7c044e3e33a9 f (no-eol)
+  $ hg histedit -r '.'
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved