Fix `bsdtar` in Git for Windows' SDKs

[dscho]

@dennisameling reported that a recent MSYS2 upgrade of OpenSSL to v3.x broke bsdtar, which we need to build Pacman packages (including mingw-w64-git, the core package around which Git for Windows is built).

Let's address this in the same way as we did when we had to downgrade bash.

Previously, in such cases @vdye or I identified which version needed to be downgraded to, downloaded it from https://repo.msys2.org/, installed it in a local git-sdk-* clone using pacman -U, preventing it from being upgraded by editing /etc/pacman.conf's IgnorePkg line (and uncommenting it), committing and pushing.

This is a bit fragile and might not even work if a previous package was removed from MSYS2's Pacman repository (e.g. to save space). An alternative (that we had not tried before) would be to use only information already in the git-sdk-* commit history to downgrade.

We could use the opportunity to automate this. What I currently have in mind is a shell script in build-extra that takes two arguments, a package name (or even a list of package names) and a shell script snippet verifying that the downgrading fixes the underlying issue, e.g. bsdtar -h || test $? = 1). The script would:

• identify the current package version (by expanding the /var/lib/pacman/local/$PACKAGE_NAME-[0-9]* wildcard pointing to a directory)
• identify the git-sdk-* commit that added said directory
• remove the files of the package (the list of files is contained in a file in that directory, tree or mtree IIRC)
• run the post_remove (or pre_upgrade, or however it's called) scriptlet of the package if there is any (contained as shell function in the install script in the directory)
• reinstate the files of the previous package version (looking at the information of the parent commit)
• run the post_upgrade scriptlet for that version
• verify that the underlying issue is fixed
• commit the files and the package directory

This could be done in a partial clone (probably even shallow, with a depth of 500 or 1000) so that it can be run efficiently in a GitHub workflow.

/cc'ing @ldennington and @jeffhostetler (because I'm mostly off of the computer until the second week of January)

[dennisameling]

Looks like downgrading bsdtar only is not enough: https://github.com/git-for-windows/git-sdk-64/actions/runs/3771072281/jobs/6411273717. A total of 9 packages were updated, so I'll probably just add all of them to IgnorePkg for now.

[dennisameling]

Alright, this should fix things for now in git-sdk-64. Currently waiting for CI to do its thing and checking some things manually as well.

[dennisameling]

Yep, that worked 🎉 @dscho ok to merge git-for-windows/git-sdk-64#57?

[rimrul]

@dscho, we'll probably need a way to prevent pacman updates from undoing this (see git-for-windows/git-sdk-64@b635d19)