Initial test to assure we don't replace objects by default (#364)

It will be followed up by supporting it on various levels, which is
more complex than it sounds.

Author: Byron

git-odb/tests/fixtures/make_replaced_history.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+set -eu -o pipefail
+
+git init -q
+git config commit.gpgsign false
+
+echo "#include <stdio.h>" > file.c && git add file.c
+git commit -m "Initial commit"
+echo "// 2nd line" >> file.c && git commit -am "2nd commit"
+echo "// 3rd line" >> file.c && git commit -am "3rd commit"
+echo "// 4th line" >> file.c && git commit -am "4th commit"
+git branch long_history HEAD^ # Create branch off of 3rd commit
+new_base=$(echo 'Short history stops here' | git commit-tree 'HEAD~2^{tree}')
+git rebase --onto $new_base HEAD~2
+git replace HEAD~1 long_history # Here's the git-replace

git-odb/tests/odb/store/dynamic.rs

@@ -204,6 +204,27 @@ fn write() -> crate::Result {
     Ok(())
 }
 
+#[test]
+fn object_replacement() {
+    let dir = git_testtools::scripted_fixture_repo_read_only("make_replaced_history.sh").unwrap();
+    let handle = git_odb::at(dir.join(".git/objects")).unwrap();
+    let mut buf = Vec::new();
+    let third_commit = handle
+        .find_commit(hex_to_id("434e5a872d6738d1fffd1e11e52a1840b73668c6"), &mut buf)
+        .unwrap();
+
+    let orphan_of_new_history = hex_to_id("0703c317e28068f39834ae61e7ab941b7d672322");
+    assert_eq!(
+        third_commit.parents().collect::<Vec<_>>(),
+        vec![orphan_of_new_history],
+        "no replacements are known by default, hence this is the replaced commit, not the replacement"
+    );
+
+    drop(third_commit);
+    let orphan = handle.find_commit(orphan_of_new_history, &mut buf).unwrap();
+    assert_eq!(orphan.parents.len(), 0);
+}
+
 #[test]
 fn contains() {
     let handle = db();