diff --git a/rhodecode/controllers/compare.py b/rhodecode/controllers/compare.py
--- a/rhodecode/controllers/compare.py
+++ b/rhodecode/controllers/compare.py
@@ -89,11 +89,17 @@ class CompareController(BaseRepoControll
         # other_ref will be evaluated in other_repo
         other_ref = (other_ref_type, other_ref)
         other_repo = request.GET.get('other_repo', org_repo)
+        # If merge is True:
+        #   Show what org would get if merged with other:
+        #   List changesets that are ancestors of other but not of org.
+        #   New changesets in org is thus ignored.
+        #   Diff will be from common ancestor, and merges of org to other will thus be ignored.
+        # If merge is False:
+        #   Make a raw diff from org to other, no matter if related or not.
+        #   Changesets in one and not in the other will be ignored
+        merge = bool(request.GET.get('merge'))
         # fulldiff disables cut_off_limit
         c.fulldiff = request.GET.get('fulldiff')
-        # only consider this range of changesets
-        rev_start = request.GET.get('rev_start')
-        rev_end = request.GET.get('rev_end')
         # partial uses compare_cs.html template directly
         partial = request.environ.get('HTTP_X_PARTIAL_XHR')
         # as_form puts hidden input field with changeset revisions
@@ -103,7 +109,8 @@ class CompareController(BaseRepoControll
             repo_name=other_repo,
             org_ref_type=other_ref[0], org_ref=other_ref[1],
             other_repo=org_repo,
-            other_ref_type=org_ref[0], other_ref=org_ref[1])
+            other_ref_type=org_ref[0], other_ref=org_ref[1],
+            merge=merge or '')
 
         org_repo = Repository.get_by_repo_name(org_repo)
         other_repo = Repository.get_by_repo_name(other_repo)
@@ -133,37 +140,23 @@ class CompareController(BaseRepoControll
         c.org_ref_type = org_ref[0]
         c.other_ref_type = other_ref[0]
 
-        if rev_start and rev_end:
-            # swap revs with cherry picked ones, save them for display
-            #org_ref = ('rev', rev_start)
-            #other_ref = ('rev', rev_end)
-            c.org_ref = rev_start[:12]
-            c.other_ref = rev_end[:12]
-            # get parent of
-            # rev start to include it in the diff
-            _cs = other_repo.scm_instance.get_changeset(rev_start)
-            rev_start = _cs.parents[0].raw_id if _cs.parents else EmptyChangeset().raw_id
-            org_ref = ('rev', rev_start)
-            other_ref = ('rev', rev_end)
-            #if we cherry pick it's not remote, make the other_repo org_repo
-            org_repo = other_repo
-
-        c.cs_ranges, ancestor = PullRequestModel().get_compare_data(
-            org_repo, org_ref, other_repo, other_ref)
+        c.cs_ranges, c.ancestor = PullRequestModel().get_compare_data(
+            org_repo, org_ref, other_repo, other_ref, merge)
 
         c.statuses = c.rhodecode_db_repo.statuses([x.raw_id for x in
                                                    c.cs_ranges])
         if partial:
+            assert c.ancestor
             return render('compare/compare_cs.html')
 
-        if ancestor and org_repo != other_repo:
+        if c.ancestor:
+            assert merge
             # case we want a simple diff without incoming changesets,
             # previewing what will be merged.
-            # Make the diff on the forked repo, with
-            # revision that is common ancestor
+            # Make the diff on the other repo (which is known to have other_ref)
             log.debug('Using ancestor %s as org_ref instead of %s'
-                      % (ancestor, org_ref))
-            org_ref = ('rev', ancestor)
+                      % (c.ancestor, org_ref))
+            org_ref = ('rev', c.ancestor)
             org_repo = other_repo
 
         diff_limit = self.cut_off_limit if not c.fulldiff else None