notes: Add git_note_commit_read

This also adds tests for this function.

include/git2/notes.h

@@ -94,6 +94,25 @@ GIT_EXTERN(int) git_note_read(
 	const char *notes_ref,
 	const git_oid *oid);
 
+
+/**
+ * Read the note for an object from a note commit
+ *
+ * The note must be freed manually by the user.
+ *
+ * @param out pointer to the read note; NULL in case of error
+ * @param repo repository where to look up the note
+ * @param notes_commit a pointer to the notes commit object
+ * @param oid OID of the git object to read the note from
+ *
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_note_commit_read(
+	git_note **out,
+	git_repository *repo,
+	git_commit *notes_commit,
+	const git_oid *oid);
+
 /**
  * Get the note author
  *

src/notes.c

@@ -463,6 +463,28 @@ int git_note_read(git_note **out, git_repository *repo,
 	return error;
 }
 
+int git_note_commit_read(
+	git_note **out,
+	git_repository *repo,
+	git_commit *notes_commit,
+	const git_oid *oid)
+{
+	int error;
+	git_tree *tree = NULL;
+	char target[GIT_OID_HEXSZ + 1];
+
+	git_oid_tostr(target, sizeof(target), oid);
+
+	if ((error = git_commit_tree(&tree, notes_commit)) < 0)
+		goto cleanup;
+
+	error = note_lookup(out, repo, notes_commit, tree, target);
+
+cleanup:
+	git_tree_free(tree);
+	return error;
+}
+
 int git_note_create(
 	git_oid *out,
 	git_repository *repo,

tests/notes/notes.c

@@ -375,6 +375,46 @@ static char *messages[] = {
 
 #define MESSAGES_COUNT (sizeof(messages)/sizeof(messages[0])) - 1
 
+/* Test that we can read a note */
+void test_notes_notes__can_read_a_note(void)
+{
+	git_oid note_oid, target_oid;
+	git_note *note;
+
+	create_note(&note_oid, "refs/notes/i-can-see-dead-notes", "4a202b346bb0fb0db7eff3cffeb3c70babbd2045", "I decorate 4a20\n");
+
+	cl_git_pass(git_oid_fromstr(&target_oid, "4a202b346bb0fb0db7eff3cffeb3c70babbd2045"));
+
+	cl_git_pass(git_note_read(&note, _repo, "refs/notes/i-can-see-dead-notes", &target_oid));
+
+	cl_assert_equal_s(git_note_message(note), "I decorate 4a20\n");
+
+	git_note_free(note);
+}
+
+/* Test that we can read a note with from commit api */
+void test_notes_notes__can_read_a_note_from_a_commit(void)
+{
+	git_oid oid, notes_commit_oid;
+	git_commit *notes_commit;
+	git_note *note;
+
+	cl_git_pass(git_oid_fromstr(&oid, "4a202b346bb0fb0db7eff3cffeb3c70babbd2045"));
+
+	cl_git_pass(git_note_commit_create(&notes_commit_oid, NULL, _repo, NULL, _sig, _sig, &oid, "I decorate 4a20\n", 1));
+
+	git_commit_lookup(&notes_commit, _repo, &notes_commit_oid);
+
+	cl_assert(notes_commit);
+
+	cl_git_pass(git_note_commit_read(&note, _repo, notes_commit, &oid));
+
+	cl_assert_equal_s(git_note_message(note), "I decorate 4a20\n");
+
+	git_commit_free(notes_commit);
+	git_note_free(note);
+}
+
 /*
  * $ git ls-tree refs/notes/fanout
  * 040000 tree 4b22b35d44b5a4f589edf3dc89196399771796ea    84