cmd/dist: more robust cleanup

cmd/dist: more robust cleanup

Identify generated files by name prefix (z*) and content
(^// Code generated by go tool dist)
instead of having a fixed list. This will be more robust
against doing make.bash and then rewinding git and
then doing make.bash again.

Change-Id: If9b4d02f5ad65345623866176d96e9894a957dc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/501036
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>