Refactor "change file" API (#34855)

Follow up the "editor" refactor, use the same approach to simplify code,
and fix some docs & comments

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

routers/web/repo/editor.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 
 	git_model "code.gitea.io/gitea/models/git"
+	"code.gitea.io/gitea/models/issues"
 	"code.gitea.io/gitea/models/unit"
 	"code.gitea.io/gitea/modules/charset"
 	"code.gitea.io/gitea/modules/git"
@@ -138,6 +139,11 @@ func prepareEditorCommitSubmittedForm[T forms.CommitCommonFormInterface](ctx *co
 		return nil
 	}
 
+	if !issues.CanMaintainerWriteToBranch(ctx, ctx.Repo.Permission, targetBranchName, ctx.Doer) {
+		ctx.NotFound(nil)
+		return nil
+	}
+
 	// Committer user info
 	gitCommitter, valid := WebGitOperationGetCommitChosenEmailIdentity(ctx, commonForm.CommitEmail)
 	if !valid {