Implement support for Git hashing with SHA-256

SHA-256 is Git's next hash algorithm. The world is still basically stuck
on SHA-1 with git, but shouldn't be. We can at least do our part to get
ready.

On the C++ implementation side, only a little bit of generalization was
needed, and that was fairly straight-forward. The tests (unit and
system) were actually bigger, and care was taken to make sure they were
all cover both algorithms equally.

src/libutil-tests/data/git/check-data.sh

@@ -2,30 +2,34 @@
 
 set -eu -o pipefail
 
-export TEST_ROOT=$(realpath ${TMPDIR:-/tmp}/nix-test)/git-hashing/check-data
-mkdir -p $TEST_ROOT
+TEST_ROOT=$(realpath "${TMPDIR:-/tmp}/nix-test")/git-hashing/check-data
+export TEST_ROOT
+mkdir -p "$TEST_ROOT"
 
-repo="$TEST_ROOT/scratch"
-git init "$repo"
+for hash in sha1 sha256; do
+  repo="$TEST_ROOT/scratch-$hash"
+  git init "$repo" --object-format="$hash"
 
-git -C "$repo" config user.email "you@example.com"
-git -C "$repo" config user.name "Your Name"
+  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=$(sha1sum -b < "./hello-world-blob.bin" | head -c 40)
+  # `-w` to write for tree test
+  freshlyAddedHash=$(git -C "$repo" hash-object -w -t blob --stdin < "./hello-world.bin")
+  encodingHash=$("${hash}sum" -b < "./hello-world-blob.bin" | sed 's/ .*//')
 
-# If the hashes match, then `hello-world-blob.bin` must be the encoding
-# of `hello-world.bin`.
-[[ "$encodingHash" == "$freshlyAddedHash" ]]
+  # 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 "$repo" hash-object -w -t tree --stdin
+  # Create empty directory object for tree test
+  echo -n | git -C "$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=$(sha1sum -b < "./tree.bin" | head -c 40)
+  # Relies on both child hashes already existing in the git store
+  tree=tree-${hash}
+  freshlyAddedHash=$(git -C "$repo" mktree < "${tree}.txt")
+  encodingHash=$("${hash}sum" -b < "${tree}.bin" | sed 's/ .*//')
 
-# 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" ]]
+  # 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" ]]
+done

src/libutil-tests/data/git/tree-sha256.txt

@@ -0,0 +1,4 @@
+100644 blob ce60f5ad78a08ac24872ef74d78b078f077be212e7a246893a1a5d957dfbc8b1	Foo
+100755 blob ce60f5ad78a08ac24872ef74d78b078f077be212e7a246893a1a5d957dfbc8b1	bAr
+040000 tree 6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321	baZ
+120000 blob ce60f5ad78a08ac24872ef74d78b078f077be212e7a246893a1a5d957dfbc8b1	quuX