histedit: check mutability of contexts correctly

The right way to check if a context is mutable is to call .mutable(),
not to compare .phase() with public.

hgext/histedit.py

@@ -751,7 +751,7 b' def between(repo, old, new, keep):'
         if repo.revs('(%ld) and merge()', ctxs):
             raise util.Abort(_('cannot edit history that contains merges'))
         root = ctxs[0] # list is already sorted by repo.set
-        if not root.phase():
+        if not root.mutable():
             raise util.Abort(_('cannot edit immutable changeset: %s') % root)
     return [c.node() for c in ctxs]