cmd/go: fix svn vctest on Windows and make them timezone agnostic

cmd/go: fix svn vctest on Windows and make them timezone agnostic

This CL updates svn vctest so they work on Windows.

Side effect is that svn logs are no longer timezone dependant, as this
updated the `svn log` command to format logs using XML, which contain
UTC dates instead of local dates. Therefore it supersedes
https://go-review.googlesource.com/c/go/+/447795.

Fixes #56555
Fixes #56527

Change-Id: I5a654fd0a0f741e0a8a6ca7021ad699c07b974d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/447935
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>