Fix commit SHA not showing in merge request compare dropdown

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17011
in GitLab 11.1 refactored the merge request diff comparison
functionality but omitted the commit SHA due to a change in
the API (truncated_commit_sha -> short_commit_sha).

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55822

app/assets/javascripts/diffs/components/compare_versions_dropdown.vue

@@ -129,7 +129,7 @@ export default {
                 </strong>
               </div>
               <div>
-                <small class="commit-sha"> {{ version.truncated_commit_sha }} </small>
+                <small class="commit-sha"> {{ version.short_commit_sha }} </small>
               </div>
               <div>
                 <small>

changelogs/unreleased/sh-fix-issue-55822.yml

@@ -0,0 +1,5 @@
+---
+title: Fix commit SHA not showing in merge request compare dropdown
+merge_request: 24084
+author:
+type: fixed

spec/features/merge_request/user_sees_versions_spec.rb

@@ -64,6 +64,17 @@ describe 'Merge request > User sees versions', :js do
       end
     end
 
+    it 'shows the commit SHAs for every version in the dropdown' do
+      page.within '.mr-version-dropdown' do
+        find('.btn-default').click
+
+        page.within('.dropdown-content') do
+          shas = merge_request.merge_request_diffs.map { |diff| Commit.truncate_sha(diff.head_commit_sha) }
+          shas.each { |sha| expect(page).to have_content(sha) }
+        end
+      end
+    end
+
     it 'shows comments that were last relevant at that version' do
       expect(page).to have_content '5 changed files'