Fix bug automerge cannot be chosed when there is only 1 merge style (#33040) (#33043)

Backport #33040 by @lunny

This is a quick bug fix. Even if there is only 1 merge style, the
dropdown menu will still be displayed to allow users to choose
automerge.

Fix #32448

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

web_src/js/components/PullRequestMergeForm.vue

@@ -147,7 +147,7 @@ function clearMergeMessage() {
             </template>
           </span>
         </button>
-        <div class="ui dropdown icon button" @click.stop="showMergeStyleMenu = !showMergeStyleMenu" v-if="mergeStyleAllowedCount>1">
+        <div class="ui dropdown icon button" @click.stop="showMergeStyleMenu = !showMergeStyleMenu">
           <svg-icon name="octicon-triangle-down" :size="14"/>
           <div class="menu" :class="{'show':showMergeStyleMenu}">
             <template v-for="msd in mergeForm.mergeStyles">