When a Git repository is using split index files, onefetch (2.10.2) fails with an error. ### Steps to reproduce ``` repo=$(mktemp -d) git -c core.splitIndex=true init $repo git -C $repo commit --allow-empty -m init env -C $repo onefetch ``` ### Example output ``` Initialized empty Git repository in /run/user/1000/tmp.qTh7DooE0d/.git/ [master (root-commit) 87171b0] init [onefetch error]: unsupported mandatory extension: 'link'; class=Index (10) ```