rebase: check for conflicts before continuing

When there are unresolved merge conflicts, there is no reason
to make the user wait for rebase to process all of the
already rebased commits just to complain that it cannot do
anything. Abort early.

hgext/rebase.py

@@ -661,6 +661,9 def rebase(ui, repo, **opts):
                     _('abort and continue do not allow specifying revisions'))
             if abortf and opts.get('tool', False):
                 ui.warn(_('tool option will be ignored\n'))
+            if contf:
+                ms = mergemod.mergestate.read(repo)
+                cmdutil.checkunresolved(ms)
 
             retcode = rbsrt._prepareabortorcontinue(abortf)
             if retcode is not None:

tests/test-rebase-conflicts.t

@@ -73,8 +73,6 Conflicting rebase:
 Try to continue without solving the conflict:
 
   $ hg rebase --continue
-  already rebased 3:3163e20567cc "L1" as 3e046f2ecedb
-  rebasing 4:46f0b057b5c0 "L2"
   abort: unresolved merge conflicts (see 'hg help resolve')
   [255]