rebase: rename and test '_destrebase'

We make the name consistent with the other similar revsets and make sure it has
minimal tests.

hgext/rebase.py

@@ -69,7 +69,7 b' def _destrebase(repo):'
     branch = repo[None].branch()
     return repo[branch].rev()
 
-def _rebasedefaultdest(repo, subset, x):
+def _revsetdestrebase(repo, subset, x):
     # ``_rebasedefaultdest()``
 
     # default destination for rebase.
@@ -1222,4 +1222,4 b' def uisetup(ui):'
          _("use 'hg rebase --continue' or 'hg rebase --abort'")])
     # ensure rebased rev are not hidden
     extensions.wrapfunction(repoview, '_getdynamicblockers', _rebasedvisible)
-    revset.symbols['_rebasedefaultdest'] = _rebasedefaultdest
+    revset.symbols['_destrebase'] = _revsetdestrebase

tests/test-rebase-scenario-global.t

@@ -743,3 +743,14 b' Test that rebase is not confused by $CWD'
   rebasing 2:779a07b1b7a0 "first source commit"
   rebasing 3:a7d6f3a00bf3 "second source with subdir" (tip)
   saved backup bundle to $TESTTMP/cwd-vanish/.hg/strip-backup/779a07b1b7a0-853e0073-backup.hg (glob)
+
+Test experimental revset
+
+  $ cd ..
+  $ hg log -r '_destrebase()'
+  changeset:   3:1910d5ff34ea
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     second source with subdir
+