rebase: enable multidest by default

This was intended to be done by D470. But there was a minor documentation
issue. The feature is quite usable now so it gets formally documented and
enabled.

There is no behavior change for people not using the `SRC` or `ALLSRC` in
rebase destination revset.

.. feature:: Rebase with different destination per source revision

  Previously, rebase only supports one unique destination. Now ``SRC`` and
  ``ALLSRC`` can be used in rebase destination revset to precisely define
  destination per each individual source revision.

  For example, the following command could move some orphaned changesets to
  reasonable new places so they become no longer orphaned::

    hg rebase
      -r 'orphan()-obsolete()'
      -d 'max((successors(max(roots(ALLSRC) & ::SRC)^)-obsolete())::)'

Differential Revision: https://phab.mercurial-scm.org/D1063

hgext/rebase.py

@@ -643,6 +643,11 b' def rebase(ui, repo, **opts):'
       4. If you do not specify any of ``--rev``, ``source``, or ``--base``,
          rebase will use ``--base .`` as above.
 
+    If ``--source`` or ``--rev`` is used, special names ``SRC`` and ``ALLSRC``
+    can be used in ``--dest``. Destination would be calculated per source
+    revision with ``SRC`` substituted by that single source revision and
+    ``ALLSRC`` substituted by all source revisions.
+
     Rebase will destroy original changesets unless you use ``--keep``.
     It will also move your bookmarks (even if you do).
 
@@ -691,6 +696,12 b' def rebase(ui, repo, **opts):'
 
           hg rebase -r "branch(featureX)" -d 1.3 --keepbranches
 
+      - stabilize orphaned changesets so history looks linear::
+
+          hg rebase -r 'orphan()-obsolete()'\
+ -d 'first(max((successors(max(roots(ALLSRC) & ::SRC)^)-obsolete())::) +\
+ max(::((roots(ALLSRC) & ::SRC)^)-obsolete()))'
+
     Configuration Options:
 
     You can make rebase require a destination if you set the following config
@@ -884,8 +895,6 b' def _definedestmap(ui, repo, destf=None,'
             # fast path: try to resolve dest without SRC alias
             dest = scmutil.revsingle(repo, destf, localalias=alias)
         except error.RepoLookupError:
-            if not ui.configbool('experimental', 'rebase.multidest'):
-                raise
             # multi-dest path: resolve dest for each SRC separately
             destmap = {}
             for r in rebaseset:

mercurial/configitems.py

@@ -532,9 +532,6 b" coreconfigitem('experimental', 'mergedri"
 coreconfigitem('experimental', 'obsmarkers-exchange-debug',
     default=False,
 )
-coreconfigitem('experimental', 'rebase.multidest',
-    default=False,
-)
 coreconfigitem('experimental', 'remotenames',
     default=False,
 )

tests/test-rebase-dest.t

@@ -115,7 +115,6 b' Setup rebase with multiple destinations'
   > [extensions]
   > maprevset=$TESTTMP/maprevset.py
   > [experimental]
-  > rebase.multidest=true
   > evolution=true
   > EOF