A fresh `git init` in detachGitDir dropped core.sparseCheckout and the
sparse-checkout patterns, and rebuilding the index via write-tree/
read-tree discarded skip-worktree bits. Files intentionally absent from
a sparse working tree then read as deletions, which delta capture could
apply back to the parent.
detachGitDir now restores the index verbatim (preserving skip-worktree,
assume-unchanged, and exact stage entries) and carries
core.sparseCheckout, core.sparseCheckoutCone, and info/sparse-checkout
into the detached .git before restoring the index. Falls back to
read-tree HEAD only when the source had no index.
Fixes#6003
roboomp
970043bd8a