pull-requests: show common ancestor inside pull-request view.

- common ancestors helps to determine what is a base of compare.

rhodecode/controllers/pullrequests.py

@@ -684,7 +684,6 b' class PullrequestsController(BaseRepoCon'
         c.shadow_clone_url = PullRequestModel().get_shadow_clone_url(
             pull_request_at_ver)
 
-        c.ancestor = None  # empty ancestor hidden in display
         c.pull_request = pull_request_display_obj
         c.pull_request_latest = pull_request_latest
 
@@ -801,12 +800,15 b' class PullrequestsController(BaseRepoCon'
         target_commit = EmptyCommit()
         c.missing_requirements = False
 
+        source_scm = source_repo.scm_instance()
+        target_scm = target_repo.scm_instance()
+
         # try first shadow repo, fallback to regular repo
         try:
             commits_source_repo = pull_request_latest.get_shadow_repo()
         except Exception:
             log.debug('Failed to get shadow repo', exc_info=True)
-            commits_source_repo = source_repo.scm_instance()
+            commits_source_repo = source_scm
 
         c.commits_source_repo = commits_source_repo
         commit_cache = {}
@@ -830,6 +832,15 b' class PullrequestsController(BaseRepoCon'
                 'Failed to get all required data from repo', exc_info=True)
             c.missing_requirements = True
 
+        c.ancestor = None  # set it to None, to hide it from PR view
+
+        try:
+            ancestor_id = source_scm.get_common_ancestor(
+                source_commit.raw_id, target_commit.raw_id, target_scm)
+            c.ancestor_commit = source_scm.get_commit(ancestor_id)
+        except Exception:
+            c.ancestor_commit = None
+
         c.statuses = source_repo.statuses(
             [x.raw_id for x in c.commit_ranges])
 

rhodecode/templates/pullrequests/pullrequest_show.mako

@@ -83,6 +83,11 b''
                     <span class="clone-url">
                         <a href="${h.url('summary_home', repo_name=c.pull_request.source_repo.repo_name)}">${c.pull_request.source_repo.clone_url()}</a>
                     </span>
+                    <br/>
+                    % if c.ancestor_commit:
+                        ${_('Common ancestor')}:
+                        <code><a href="${h.url('changeset_home', repo_name=c.target_repo.repo_name, revision=c.ancestor_commit.raw_id)}">${h.show_id(c.ancestor_commit)}</a></code>
+                    % endif
                 </div>
                 <div class="pr-pullinfo">
                      %if h.is_hg(c.pull_request.source_repo):