rebase: remove experimental option from 'rebase' config section

Changeset f0e9f38d250f introduced a guard against case where obsolete changesets
are included in the rebase in a way this will result in divergence (because
rebase create new successors for changeset which already have successors). In
the same go a 'rebase.allowdivergence' option was introduced to control that
behavior.

We rename this config option to 'experimental.allowdivergence' for multiple
reasons:

* First this behavior is attached to changeset evolution, a feature still
 experimental.

* Second, there was no 'rebase' section in config before we introduced this
  option. I would like to avoid proliferation of micro config section and
  therefore would like to avoid the creation of this new section just for an
  experimental feature.

* Third, this guard (warning the user about a history rewriting operation that
  will create divergence) will very likely be generalised to all history
  rewriting operations, making this not rebase specific.

* Finally, because this will likely be a general guard present a bit everywhere
  in the UI we'll likely end up with something better than a config option to
  control this behavior, so having the current config option living in
  experimental will allow us make it disappear in the future.

So we banish this config option back to the experimental section where it
belongs, killing the newly born 'rebase' config section in the process.

hgext/rebase.py

@@ -306,7 +306,7 b' def rebase(ui, repo, **opts):'
                 rebaseobsskipped = set(obsoletenotrebased)
 
                 # Obsolete node with successors not in dest leads to divergence
-                divergenceok = ui.configbool('rebase',
+                divergenceok = ui.configbool('experimental',
                                              'allowdivergence')
                 divergencebasecandidates = rebaseobsrevs - rebaseobsskipped
 
@@ -316,7 +316,7 b' def rebase(ui, repo, **opts):'
                     msg = _("this rebase will cause "
                             "divergences from: %s")
                     h = _("to force the rebase please set "
-                          "rebase.allowdivergence=True")
+                          "experimental.allowdivergence=True")
                     raise error.Abort(msg % (",".join(divhashes),), hint=h)
 
                 # - plain prune (no successor) changesets are rebased

tests/test-rebase-obsolete.t

@@ -772,7 +772,7 b' If a rebase is going to create divergenc'
   unstable: 1 changesets
   $ hg rebase -s 10 -d 12
   abort: this rebase will cause divergences from: 121d9e3bc4c6
-  (to force the rebase please set rebase.allowdivergence=True)
+  (to force the rebase please set experimental.allowdivergence=True)
   [255]
   $ hg log -G
   @  15:73568ab6879d bar foo
@@ -791,9 +791,9 b' If a rebase is going to create divergenc'
   |
   o  0:4a2df7238c3b A
   
-With rebase.allowdivergence=True, rebase can create divergence
+With experimental.allowdivergence=True, rebase can create divergence
 
-  $ hg rebase -s 10 -d 12 --config rebase.allowdivergence=True
+  $ hg rebase -s 10 -d 12 --config experimental.allowdivergence=True
   rebasing 10:121d9e3bc4c6 "P"
   rebasing 15:73568ab6879d "bar foo" (tip)
   $ hg summary