blog: use first OldURL to generate atom feed ID
blog: use first OldURL to generate atom feed ID

This way, renaming an article does not make it reappear
in an RSS reader as a new article.

Change-Id: I4e92707f9ea4983bf8506cde826d5e5f48a3b36f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/223751
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools