Trim extra period when merge error displayed

When a merge error happens, previously the UI displayed two periods:

```
Merge failed: Failed to squash. Should be done manually.. Please try
again.
```

This happened because the backend sent the error message, "Failed to
squash. Should be done manually.", while the frontend added an extra
period.

To avoid this issue, we now trim the period and add it in the sentence.

app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue

@@ -121,8 +121,14 @@ export default {
       );
     },
     mergeError() {
+      let { mergeError } = this.mr;
+
+      if (mergeError && mergeError.slice(-1) === '.') {
+        mergeError = mergeError.slice(0, -1);
+      }
+
       return sprintf(s__('mrWidget|Merge failed: %{mergeError}. Please try again.'), {
-        mergeError: this.mr.mergeError,
+        mergeError,
       });
     },
   },

changelogs/unreleased/sh-trim-extra-periods-merge-error.yml

+---
+title: Trim extra period when merge error displayed
+merge_request: 23737
+author:
+type: fixed

spec/features/merge_request/user_merges_when_pipeline_succeeds_spec.rb

@@ -152,7 +152,7 @@ describe 'Merge request > User merges when pipeline succeeds', :js do
       before do
         merge_request.update(
           merge_user: merge_request.author,
-          merge_error: 'Something went wrong'
+          merge_error: 'Something went wrong.'
         )
         refresh
       end
@@ -162,7 +162,7 @@ describe 'Merge request > User merges when pipeline succeeds', :js do
         wait_for_requests
 
         page.within('.mr-section-container') do
-          expect(page).to have_content('Merge failed: Something went wrong')
+          expect(page).to have_content('Merge failed: Something went wrong. Please try again.')
         end
       end
     end
@@ -171,7 +171,7 @@ describe 'Merge request > User merges when pipeline succeeds', :js do
       before do
         merge_request.update(
           merge_user: merge_request.author,
-          merge_error: 'Something went wrong'
+          merge_error: 'Something went wrong.'
         )
         refresh
       end
@@ -181,7 +181,7 @@ describe 'Merge request > User merges when pipeline succeeds', :js do
         wait_for_requests
 
         page.within('.mr-section-container') do
-          expect(page).to have_content('Merge failed: Something went wrong')
+          expect(page).to have_content('Merge failed: Something went wrong. Please try again.')
         end
       end
     end