Refactor branch/tag selector to Vue SFC (#23421)

Follow #23394

There were many bad smells in old code. This PR only moves the code into
Vue SFC, doesn't touch the unrelated logic.

update: after
5f23218c85
, there should be no usage of the vue-rumtime-compiler anymore
(hopefully), so I think this PR could close #19851

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/js/utils/url.js

@@ -0,0 +1,3 @@
+export function pathEscapeSegments(s) {
+  return s.split('/').map(encodeURIComponent).join('/');
+}