Clarify path param naming (#32969)

In history (from some legacy frameworks), both `:name` and `name` are
supported as path path name, `:name` is an alias to `name`.

To make code consistent, now we should only use `name` but not `:name`.

Also added panic check in related functions to make sure the name won't
be abused in case some downstreams still use them.

routers/api/v1/repo/tree.go

@@ -56,7 +56,7 @@ func GetTree(ctx *context.APIContext) {
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 
-	sha := ctx.PathParam(":sha")
+	sha := ctx.PathParam("sha")
 	if len(sha) == 0 {
 		ctx.Error(http.StatusBadRequest, "", "sha not provided")
 		return