do checkout before changing HEAD

[simonbyrne]

According to the libgit2 docs, we should call git_checkout_* before changing the HEAD ref. This is generally not a big problem as Pkg does a force checkout (which is probably not a good idea either, but that's for another day), except in a few edge cases.

Fixes both #17610 and #18420. Unfortunately, I have no idea how to test this.

[tkelman]

maybe we should try to create a test repo that hits one of those corner cases with permissions or file types changing?

[simonbyrne]

I guess, but is there a way to do it so that it works across all platforms?

[tkelman]

I don't think these issues were a problem on all platforms though. I'll have to test if they occur at all on Windows.

[simonbyrne]

Given the changes to LibGit2, a backport will need to be a bit different. Do you want me to put together another PR?

[tkelman]

let's see if/when 0.5.2 ends up being necessary before spending time on it, if non-trivial

[simonbyrne]

added a test.

[tkelman]

yay, thank you!

[tkelman]

What would the backport version of this look like?

[simonbyrne]

I guess just changing the order of checkout_tree and GitReference. Try the sb/libgit2/mads-backport branch (I haven't tested it yet)

[tkelman]

Very nice, thank you, that does appear to fix the issue on 0.5. Will add the cherry-pick -x cross-reference and run that through PkgEval when I prepare 0.5.2.