[BUG] Fix diff patch operation in web UI

- The form used by the diff patch operation requires that the value of
`tree_path` is set, even though it's not used. Set it to `patch` so this
feature can be used again.
- Regression of 08fe6f8c7e.

templates/repo/editor/patch.tmpl

@@ -14,7 +14,7 @@
 					<div class="breadcrumb-divider">:</div>
 					<a class="section" href="{{$.BranchLink}}">{{.BranchName}}</a>
 					<span>{{ctx.Locale.Tr "repo.editor.or"}} <a href="{{$.BranchLink}}">{{ctx.Locale.Tr "repo.editor.cancel_lower"}}</a></span>
-					<input type="hidden" id="tree_path" name="tree_path" value="" required>
+					<input type="hidden" id="tree_path" name="tree_path" value="patch" required>
 					<input id="file-name" type="hidden" value="diff.patch">
 				</div>
 			</div>