Unable to build projects after updating to 1.46.0 when our repo is using sparse-checkouts

[photex]

Problem
Since updating to 1.46.0, for some reason we can't build our application if a git clone is using sparse checkouts.

Steps

c:\projects>cargo init foo

c:\projects>cd foo\                                                                                                                                                                                                                                                                                                                         

c:\projects\foo>git init                                                                                                                                              Initialized empty Git repository in C:/projects/foo/.git/                                                                                                                                                                                                                                                                                   

c:\projects\foo>mkdir bar                                                                                                                                                                                                                                                                                                                   

c:\projects\foo>touch bar/test.txt                                                                                                                                                                                                                                                                                                          

c:\projects\foo>echo fn main() { () } > build.rs                                                                                                                                                                                                                                                                                          

c:\projects\foo>vim build.rs

c:\projects\foo>ls
bar  build.rs  Cargo.toml  src                                                                                                                                                                                                                                                                                                              

c:\projects\foo>cargo run
   Compiling foo v0.1.0 (C:\projects\foo)
    Finished dev [unoptimized + debuginfo] target(s) in 1.31s
     Running `target\x86_64-pc-windows-msvc\debug\foo.exe`
Hello, world!


c:\projects\foo>git sparse-checkout init --cone

c:\projects\foo>git sparse-checkout add src

c:\projects\foo>cargo run
error: failed to determine package fingerprint for build script for foo v0.1.0 (C:\projects\foo)

Caused by:
  failed to determine the most recently modified file in c:\projects\foo

Caused by:
  failed to determine list of files in c:\projects\foo

Caused by:
  did not expect repo at c:/projects/foo/.git/ to be bare

c:\projects\foo>

Possible Solution(s)
I have no idea. Sorry!

Notes

Output of cargo version: cargo 1.46.0 (149022b 2020-07-17)

As far as I can tell this really breaks things in a solid way. Just disabling sparse-checkout after this isn't enough to recover.

[photex]

Ah, if you edit .git\config and remove:

[extensions]
worktreeConfig = true 

Then you'll be able to build again.

[ehuss]

Posted libgit2/libgit2#5656 to start the ball rolling to fix this.

[jynnantonix]

Could we also change cargo so that it doesn't fail if it cannot understand the git repo? It doesn't matter if the repo is completely borked; as long as there is a valid Cargo.toml, cargo should be able to run the build.