Branches without parent commit cannot be merged.

app/controllers/projects/merge_requests_controller.rb

@@ -95,6 +95,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       @note_counts = Note.where(commit_id: @commits.map(&:id)).
           group(:commit_id).count
+      begin
         @diffs = compare_action.diffs
         @merge_request.title = @merge_request.source_branch.titleize.humanize
         @target_project = @merge_request.target_project
@@ -103,6 +104,9 @@ class Projects::MergeRequestsController < Projects::ApplicationController
         diff_line_count = Commit::diff_line_count(@diffs)
         @suppress_diff = Commit::diff_suppress?(@diffs, diff_line_count)
         @force_suppress_diff = @suppress_diff
+      rescue Gitlab::Satellite::BranchesWithoutParent
+        @error = "Selected branches have no common commit so they cannot be compared."
+      end
     end
   end

app/views/projects/merge_requests/_new_submit.html.haml

@@ -10,7 +10,10 @@
   %span.pull-right
     = link_to 'Change branches', new_project_merge_request_path(@project)
-
+- if @error.present?
+  .centered-light-block
+    %h4 #{@error}
+- else
   = form_for [@project, @merge_request], html: { class: "merge-request-form" } do |f|
     .panel.panel-default

lib/gitlab/satellite/compare_action.rb

 module Gitlab
   module Satellite
+    class BranchesWithoutParent < StandardError; end
+
     class CompareAction < Action
       def initialize(user, target_project, target_branch, source_project, source_branch)
         super user, target_project
@@ -22,7 +24,7 @@ module Gitlab
           diffs
         end
       rescue Grit::Git::CommandFailed => ex
-        handle_exception(ex)
+        raise BranchesWithoutParent
       end
       # Retrieve an array of commits between the source and the target