Fix incorrect tree path value for patch editor (#29377) (#29421)

Backport #29377 by wxiaoguang

Regression of #18718. When submitting the form,
EditRepoFileForm.TreePath is marked as "Required", so the value can't be
empty. The value is not used by backend, so use a meaningful dummy value
for it.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/editor/patch.tmpl

@@ -14,8 +14,8 @@
 					<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 id="file-name" maxlength="500" type="hidden" value="diff.patch">
+					<input type="hidden" name="tree_path" value="__dummy_for_EditRepoFileForm.TreePath(Required)__">
+					<input id="file-name" type="hidden" value="diff.patch">
 				</div>
 			</div>
 			<div class="field">