Reduce parent stat-per-file to stat-per-dir
Reduce parent stat-per-file to stat-per-dir

This is an installation microoptimisation for ~ 3% performance during
unpack performance: stat() calls are not free, involving path traversal,
ACL checking and so forth.

Before: 34s consistently to install rust-docs.
After: 32s-33s to install rust-docs.

A better still approach would be to handle creation errors by creating
the containing directories just in time, but that requires replacing
the tar extraction logic, which I haven't yet figured out how to layer
in cleanly.

inejge/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,inejge/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,inejge/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs