rebase: move destination computation in a revset

This is the first step toward making the "default destination" logic more clear
and unified. The revset is private because I'm happy to delay the bikeshedding until
after the clean up happened.

hgext/rebase.py

@@ -16,7 +16,7 b' http://mercurial.selenic.com/wiki/Rebase'
 
 from mercurial import hg, util, repair, merge, cmdutil, commands, bookmarks
 from mercurial import extensions, patch, scmutil, phases, obsolete, error
-from mercurial import copies, repoview
+from mercurial import copies, repoview, revset
 from mercurial.commands import templateopts
 from mercurial.node import nullrev, nullid, hex, short
 from mercurial.lock import release
@@ -54,6 +54,21 b' def _makeextrafn(copiers):'
             c(ctx, extra)
     return extrafn
 
+def _rebasedefaultdest(repo, subset, x):
+    # ``_rebasedefaultdest()``
+
+    # default destination for rebase.
+    # # XXX: Currently private because I expect the signature to change.
+    # # XXX: - taking rev as arguments,
+    # # XXX: - bailing out in case of ambiguity vs returning all data.
+    # # XXX: - probably merging with the merge destination.
+    # i18n: "_rebasedefaultdest" is a keyword
+    revset.getargs(x, 0, 0, _("_rebasedefaultdest takes no arguments"))
+    # Destination defaults to the latest revision in the
+    # current branch
+    branch = repo[None].branch()
+    return subset & revset.baseset([repo[branch].rev()])
+
 @command('rebase',
     [('s', 'source', '',
      _('rebase the specified changeset and descendants'), _('REV')),
@@ -252,12 +267,8 b' def rebase(ui, repo, **opts):'
             cmdutil.bailifchanged(repo)
 
             if not destf:
-                # Destination defaults to the latest revision in the
-                # current branch
-                branch = repo[None].branch()
-                dest = repo[branch]
-            else:
-                dest = scmutil.revsingle(repo, destf)
+                destf = '_rebasedefaultdest()'
+            dest = scmutil.revsingle(repo, destf)
 
             if revf:
                 rebaseset = scmutil.revrange(repo, revf)
@@ -1126,3 +1137,4 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