Git object hashing in libutil

This is the core functionality but just unit-tested and not yet made
part of the store layer. This is because there is some tech debt around
(a) repeated boilerplate hashing objects (b) better integration of the
new `SourceAccessor` type that needs to be cleaned up first.

Part of RFC 133

Co-Authored-By: Matthew Bauer <mjbauer95@gmail.com>
Co-Authored-By: Carlo Nucera <carlo.nucera@protonmail.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Florian Klink <flokli@flokli.de>

unit-test-data/libutil/git/check-data.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+
+set -eu -o pipefail
+
+export TEST_ROOT=$(realpath ${TMPDIR:-/tmp}/nix-test)/git-hashing/unit-test-data
+mkdir -p $TEST_ROOT
+
+repo="$TEST_ROOT/scratch"
+git init "$repo"
+
+git -C "$repo" config user.email "you@example.com"
+git -C "$repo" config user.name "Your Name"
+
+# `-w` to write for tree test
+freshlyAddedHash=$(git -C "$repo" hash-object -w -t blob --stdin < "./hello-world.bin")
+encodingHash=$(sha1sum -b < "./hello-world-blob.bin" | head -c 40)
+
+# If the hashes match, then `hello-world-blob.bin` must be the encoding
+# of `hello-world.bin`.
+[[ "$encodingHash" == "$freshlyAddedHash" ]]
+
+# Create empty directory object for tree test
+echo -n | git -C "$repo" hash-object -w -t tree --stdin
+
+# Relies on both child hashes already existing in the git store
+freshlyAddedHash=$(git -C "$repo" mktree < "./tree.txt")
+encodingHash=$(sha1sum -b < "./tree.bin" | head -c 40)
+
+# If the hashes match, then `tree.bin` must be the encoding of the
+# directory denoted by `tree.txt` interpreted as git directory listing.
+[[ "$encodingHash" == "$freshlyAddedHash" ]]

unit-test-data/libutil/git/tree.txt

@@ -0,0 +1,3 @@
+100644 blob 63ddb340119baf8492d2da53af47e8c7cfcd5eb2	Foo
+100755 blob 63ddb340119baf8492d2da53af47e8c7cfcd5eb2	bAr
+040000 tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904	baZ