Add sparse checkout support

[pakrym]

Fixes: #771

Reads but does not interpret any of V1 extensions. The only one of interest is the worktreeConfig which I didn't implement because config.worktree didn't seem to contain any information of interest.

Sample sparse repo config:

config.worktree

[core]
	sparseCheckout = true

config

[core]
	repositoryformatversion = 1
	filemode = false
	bare = false
	logallrefupdates = true
	symlinks = false
	ignorecase = true
[remote "origin"]
	url = https://github.com/Azure/azure-sdk-for-net
	fetch = +refs/heads/*:refs/remotes/origin/*
	promisor = true
	partialclonefilter = tree:0
[branch "main"]
	remote = origin
	merge = refs/heads/main
[extensions]
	worktreeConfig = true

[pakrym]

cc @tmat

[KalleOlaviNiemitalo]

How does GetUntrackedFiles work here if a tree is missing because of sparse checkout?

[pakrym]

To be honest, I didn't know that feature existed.

If I understand how GetUntrackedFiles works, it might be a requirement for source link to clone .gitignore before building.

But for the files that are not gitingored it should work fine, the index is still there.

What do you think is the best course of action here?

[tmat]

Thanks for the PR. I'll need to find time to study the implications of this change before I can review.

[hcoona]

10 months passed, will this MR merge?

[tmat]

Sorry for the delay.

[gitfool]

@tmat The github checkout action now supports partial checkout filters (actions/checkout#1396) and since this was merged can a new sourcelink package please be published to nuget to avoid breaking builds enabling partial checkout with:

error : Error reading git repository information: Unsupported repository version 1. Only versions up to 0 are supported.

🙏