Show 20 labels in dropdown instead of 5

When pressing ~ to show a list of labels, only 5 are shown. This can be
annoying for the user if there are more than 5 labels that start with
the same word but they can't see all matching labels in the autocomplete
list. This commit changes the list to show a maximum of 20 instead of 5.

app/assets/javascripts/gfm_auto_complete.js

@@ -337,6 +337,7 @@ class GfmAutoComplete {
       },
       // eslint-disable-next-line no-template-curly-in-string
       insertTpl: '${atwho-at}${title}',
+      limit: 20,
       callbacks: {
         ...this.getDefaultCallbacks(),
         beforeSave(merges) {

changelogs/unreleased/17596-show-all-matching-labels-when-using-label-quick-action.yml

+---
+title: Show 20 labels in dropdown instead of 5
+merge_request: 17596
+author:
+type: fixed

spec/frontend/fixtures/autocomplete_sources.rb

@@ -24,6 +24,10 @@ describe Projects::AutocompleteSourcesController, '(JavaScript fixtures)', type:
 
     create(:label, project: project, title: 'feature')
     create(:label, project: project, title: 'documentation')
+    create(:label, project: project, title: 'P1')
+    create(:label, project: project, title: 'P2')
+    create(:label, project: project, title: 'P3')
+    create(:label, project: project, title: 'P4')
 
     get :labels,
         format: :json,