Use the --prune flag when running git fetch origin commands
to delete remote-tracking refs in the local repositories.
This prevents branches and tags that have been deleted on
the origin server from reappearing in the GitHub mirrors.

It will take more work to get refs automatically deleted from
the mirror repositories, because the current Repo.push uses
a custom implementation rather than git push -f --mirror dest,
and it doesn't take the need to delete refs into account yet.

See https://git-scm.com/docs/git-fetch#Documentation/git-fetch.txt---prune
for information on the --prune flag. The --prune-tags flag isn't
needed because the local repositories are cloned with --mirror.

Updates golang/go#23099

Change-Id: I762b2d1365287d5dac40236310713c3df15d59d8
Reviewed-on: https://go-review.googlesource.com/c/build/+/190879
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

Use the --prune flag when running git fetch origin commands
to delete remote-tracking refs in the local repositories.
This prevents branches and tags that have been deleted on
the origin server from reappearing in the GitHub mirrors.

It will take more work to get refs automatically deleted from
the mirror repositories, because the current Repo.push uses
a custom implementation rather than git push -f --mirror dest,
and it doesn't take the need to delete refs into account yet.

See https://git-scm.com/docs/git-fetch#Documentation/git-fetch.txt---prune
for information on the --prune flag. The --prune-tags flag isn't
needed because the local repositories are cloned with --mirror.

Updates golang/go#23099

Change-Id: I762b2d1365287d5dac40236310713c3df15d59d8
Reviewed-on: https://go-review.googlesource.com/c/build/+/190879
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

In 2016, there were problems with git mirroring being slow and flaky,
as described in golang/go#16388. In order to resolve that problem,
CL 25110 added a custom mirroring implementation that effectively
split up the work done by a single git push -f --mirror dest command
into smaller batches of work. It was noted in the commit message and
updated documentation of CL 127315 that it may no longer be needed in
newer versions of git, and that the problem seemed to affect the HTTPS
transport, not so much the SSH transport.

By now, the following conditions are different:

• It's 2019.
• We're using a newer version of git.
• We're using the SSH transport (which may or may not be better than
  the HTTPS transport).
• We are mirroring only the refs/heads/* and refs/tags/* namespaces,
  not refs/changes/* which contains an order of magnitude more refs.

As a result, it should be possible to revert to a simple mirroring
implementation that uses a single git push -f --mirror dest command.
This removes complexity in our code, leveraging the git binary to do
more for us (at the expense of relinquishing tighter control and
ability to do things differently if we start encountering problems).
It resolves the issue of deleted refs not being deleted from mirrors
(golang/go#23099) without having to add even more complexity.

Debian 10 (buster) is the current stable release of Debian.
Start using it, since it comes with a newer version of git
(git version 2.20.1, compared to git version 2.11.0 in Debian 9),
which we want in hopes of git being better capable of pushing
a large number of refs without problems.

If we run into problems again, we might have to add some of the
complexity back, but hopefully that won't happen. Remove the old
and unused code since it's easy to get it back from VCS history.

Fixes golang/go#23099
Fixes golang/go#29560
Updates golang/go#16388

Change-Id: Id63419a5e4df5a87d7c9dc4be4e7e38a4849b3df
Reviewed-on: https://go-review.googlesource.com/c/build/+/190897
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>