Fix mergeUrlParams handling of `+`

**What was the issue?**
If a param value had `+`, it would be encoded as a
literal `+` instead of a space.

app/assets/javascripts/lib/utils/url_utility.js

 import { join as joinPaths } from 'path';
 
+// Returns a decoded url parameter value
+// - Treats '+' as '%20'
+function decodeUrlParameter(val) {
+  return decodeURIComponent(val.replace(/\+/g, '%20'));
+}
+
 // Returns an array containing the value(s) of the
 // of the key passed as an argument
 export function getParameterValues(sParam, url = window.location) {
@@ -30,7 +36,7 @@ export function mergeUrlParams(params, url) {
       .forEach(part => {
         if (part.length) {
           const kv = part.split('=');
-          merged[decodeURIComponent(kv[0])] = decodeURIComponent(kv.slice(1).join('='));
+          merged[decodeUrlParameter(kv[0])] = decodeUrlParameter(kv.slice(1).join('='));
         }
       });
   }

changelogs/unreleased/fe-fix-merge-url-params-with-plus.yml

+---
+title: Fix search preserving space when change branch
+merge_request: 31973
+author: minghuan lei
+type: fixed

spec/frontend/lib/utils/url_utility_spec.js

@@ -94,6 +94,12 @@ describe('URL utility', () => {
     it('adds and updates encoded params', () => {
       expect(urlUtils.mergeUrlParams({ a: '&', q: '?' }, '?a=%23#frag')).toBe('?a=%26&q=%3F#frag');
     });
+
+    it('treats "+" as "%20"', () => {
+      expect(urlUtils.mergeUrlParams({ ref: 'bogus' }, '?a=lorem+ipsum&ref=charlie')).toBe(
+        '?a=lorem%20ipsum&ref=bogus',
+      );
+    });
   });
 
   describe('removeParams', () => {