Add support for submodules in gitfs

[tianon]

For now, these emulate the support that git archive has for them (namely, that they present as empty directories, which are otherwise impossible to create/represent in Git).

[tianon]

Without this, trying to run sources.sh for my debuerreotype/debuerreotype image from the result of debuerreotype/debuerreotype#183 fails with:

template: --format:10:19: executing "--format" at <$.ArchGitChecksum>: error calling ArchGitChecksum: ".": Size("validate"): object not found

[tianon]

See also:

• https://github.com/tianon/dockerfiles/blob/1e525b23b7ba174ca62d36231cf03e736a33a093/buildkit/git-no-submodules.patch

• https://github.com/tianon/debian-moby/blob/e1598d3a45da6ab9d4a9acec6a4600cd4eee57d1/moby-engine/debian/patches/git-no-submodules.patch

[tianon]

I applied this to my own builds and it worked successfully: tianon/bashbrew-tianon-meta@b491ee4#diff-b5258a02eeb8ebad56e128d41fdf1cc6b6bb9e5523199bcb868ee7076cbf2ea1

[tianon]

I did not verify that git archive produces the exact same reproducible output, however. I will do that.

[tianon]

I'm glad I explicitly tested it! It caught a bug. 😄

See tianon/bashbrew-tianon-meta@ce8717b#diff-b5258a02eeb8ebad56e128d41fdf1cc6b6bb9e5523199bcb868ee7076cbf2ea1 for where applying the latest fixed it.

`tar-scrubber.go`

package main

import (
	"archive/tar"
	"io"
	"os"
)

func main() {
	in := tar.NewReader(os.Stdin)

	out := tar.NewWriter(os.Stdout)
	defer out.Flush() // note: flush instead of close to avoid the empty block at EOF

	var zero tar.Header

	for {
		hdr, err := in.Next()
		if err == io.EOF {
			break
		}
		if err != nil {
			panic(err)
		}
		hdr.Uid = zero.Uid
		hdr.Gid = zero.Gid
		hdr.Uname = zero.Uname
		hdr.Gname = zero.Gname
		hdr.ModTime = zero.ModTime
		hdr.AccessTime = zero.AccessTime
		hdr.ChangeTime = zero.ChangeTime
		if err := out.WriteHeader(hdr); err != nil {
			panic(err)
		}
		if _, err := io.Copy(out, in); err != nil {
			panic(err)
		}
	}
}

$ git archive --format=tar d12af8e5556e39f82082b44628288e2eb27d4c34:./ | go run ./tar-scrubber.go | sha256sum
3af10ded9c88b401f84baf4c72ad4d4dc3a72f614c4704e863b1d79e49c88faa  -

    "reproducibleGitChecksum": "3af10ded9c88b401f84baf4c72ad4d4dc3a72f614c4704e863b1d79e49c88faa",