Prevent server 500 on compare branches with no common history (#6555)

* Prevent 500 if there is no common mergebase
* Prevent creation of PR with no history

options/locale/locale_en-US.ini

@@ -684,6 +684,7 @@ editor.cannot_commit_to_protected_branch = Cannot commit to protected branch '%s
 
 commits.desc = Browse source code change history.
 commits.commits = Commits
+commits.no_commits = No commits in common. '%s' and '%s' have entirely different histories.
 commits.search = Search commits…
 commits.find = Search
 commits.search_all = All Branches