Add key for v-for in repo_commit_section

app/assets/javascripts/repo/repo_commit_section.vue

@@ -59,7 +59,7 @@ export default RepoCommitSection;
         <label class="col-md-4 control-label">Staged files ({{changedFiles.length}})</label>
         <div class="col-md-4">
           <ul class="list-unstyled">
-            <li v-for="file in changedFiles">
+            <li v-for="file in changedFiles" :key="file.id">
               <span class="help-block">
                 {{file.url}}
               </span>