Cargo Build git Split Index Bug

Cargo build is breaking when a build.rs file is present in a project with a git repo using split indexes.

This repo reproduces the issue.

Steps to Reproduce Bug

• git config --local include.path ../.gitconfig
• git reset (to trigger the index to be built using a split index)
• cargo build

error: failed to determine package fingerprint for build script for split_index_bug v0.1.0 (/.../split_index_bug)

Caused by:
  failed to determine the most recently modified file in /.../split_index_bug

Caused by:
  failed to determine list of files in /.../split_index_bug

Caused by:
  failed to open git index at /.../split_index_bug/.git/

Caused by:
  unsupported mandatory extension: 'link'; class=Index (10)