tests: add tests that exercise commit parsing

We currently do not have any test suites dedicated to parsing commits
from their raw representations. Add one based on `git_object__from_raw`
to be able to test special cases more easily.

tests/object/commit/parse.c

+#include "clar_libgit2.h"
+#include "commit.h"
+#include "object.h"
+#include "signature.h"
+
+static void assert_commit_parses(const char *data, size_t datalen,
+	const char *expected_treeid,
+	const char *expected_author,
+	const char *expected_committer,
+	const char *expected_encoding,
+	const char *expected_message,
+	size_t expected_parents)
+{
+	git_commit *commit;
+	if (!datalen)
+		datalen = strlen(data);
+	cl_git_pass(git_object__from_raw((git_object **) &commit, data, datalen, GIT_OBJ_COMMIT));
+
+	if (expected_author) {
+		git_signature *author;
+		cl_git_pass(git_signature_from_buffer(&author, expected_author));
+		cl_assert(git_signature__equal(author, commit->author));
+		cl_assert_equal_s(author->name, commit->author->name);
+		cl_assert_equal_s(author->email, commit->author->email);
+		cl_assert_equal_i(author->when.time, commit->author->when.time);
+		cl_assert_equal_i(author->when.offset, commit->author->when.offset);
+		cl_assert_equal_i(author->when.sign, commit->author->when.sign);
+		git_signature_free(author);
+	}
+
+	if (expected_committer) {
+		git_signature *committer;
+		cl_git_pass(git_signature_from_buffer(&committer, expected_committer));
+		cl_assert_equal_s(committer->name, commit->committer->name);
+		cl_assert_equal_s(committer->email, commit->committer->email);
+		cl_assert_equal_i(committer->when.time, commit->committer->when.time);
+		cl_assert_equal_i(committer->when.offset, commit->committer->when.offset);
+		cl_assert_equal_i(committer->when.sign, commit->committer->when.sign);
+		git_signature_free(committer);
+	}
+
+	if (expected_encoding)
+		cl_assert_equal_s(commit->message_encoding, expected_encoding);
+	else
+		cl_assert_equal_p(commit->message_encoding, NULL);
+
+	if (expected_message)
+		cl_assert_equal_s(commit->raw_message, expected_message);
+	else
+		cl_assert_equal_p(commit->message_encoding, NULL);
+
+	if (expected_treeid) {
+		git_oid tree_oid;
+		cl_git_pass(git_oid_fromstr(&tree_oid, expected_treeid));
+		cl_assert_equal_oid(&tree_oid, &commit->tree_id);
+	}
+
+	cl_assert_equal_i(commit->parent_ids.size, expected_parents);
+
+	git_object__free(&commit->object);
+}
+
+static void assert_commit_fails(const char *data, size_t datalen)
+{
+	git_object *object;
+	if (!datalen)
+		datalen = strlen(data);
+	cl_git_fail(git_object__from_raw(&object, data, datalen, GIT_OBJ_COMMIT));
+}
+
+void test_object_commit_parse__parsing_commit_succeeds(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8\n"
+		"author Author <author@example.com>\n"
+		"committer Committer <committer@example.com>\n"
+		"encoding Encoding\n"
+		"\n"
+		"Message";
+	assert_commit_parses(commit, 0,
+		"3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8",
+		"Author <author@example.com>",
+		"Committer <committer@example.com>",
+		"Encoding",
+		"Message", 0);
+}
+
+void test_object_commit_parse__parsing_commit_without_encoding_succeeds(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8\n"
+		"author Author <author@example.com>\n"
+		"committer Committer <committer@example.com>\n"
+		"\n"
+		"Message";
+	assert_commit_parses(commit, 0,
+		"3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8",
+		"Author <author@example.com>",
+		"Committer <committer@example.com>",
+		NULL,
+		"Message", 0);
+}
+
+void test_object_commit_parse__parsing_commit_with_multiple_authors_succeeds(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8\n"
+		"author Author1 <author@example.com>\n"
+		"author Author2 <author@example.com>\n"
+		"author Author3 <author@example.com>\n"
+		"author Author4 <author@example.com>\n"
+		"committer Committer <committer@example.com>\n"
+		"\n"
+		"Message";
+	assert_commit_parses(commit, 0,
+		"3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8",
+		"Author1 <author@example.com>",
+		"Committer <committer@example.com>",
+		NULL,
+		"Message", 0);
+}
+
+void test_object_commit_parse__parsing_commit_with_multiple_committers_succeeds(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8\n"
+		"author Author <author@example.com>\n"
+		"committer Committer1 <committer@example.com>\n"
+		"committer Committer2 <committer@example.com>\n"
+		"committer Committer3 <committer@example.com>\n"
+		"committer Committer4 <committer@example.com>\n"
+		"\n"
+		"Message";
+	assert_commit_parses(commit, 0,
+		"3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8",
+		"Author <author@example.com>",
+		"Committer1 <committer@example.com>",
+		NULL,
+		"Message", 0);
+}
+
+void test_object_commit_parse__parsing_commit_without_message_succeeds(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8\n"
+		"author Author <author@example.com>\n"
+		"committer Committer <committer@example.com>\n";
+	assert_commit_parses(commit, 0,
+		"3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8",
+		"Author <author@example.com>",
+		"Committer <committer@example.com>",
+		NULL,
+		"", 0);
+}
+
+void test_object_commit_parse__parsing_commit_with_unknown_fields_succeeds(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8\n"
+		"author Author <author@example.com>\n"
+		"committer Committer <committer@example.com>\n"
+		"foo bar\n"
+		"more garbage\n"
+		"\n"
+		"Message";
+	assert_commit_parses(commit, 0,
+		"3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8",
+		"Author <author@example.com>",
+		"Committer <committer@example.com>",
+		NULL,
+		"Message", 0);
+}
+
+void test_object_commit_parse__parsing_commit_with_invalid_tree_fails(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1xxx5f3445895b71d9cb0f8\n"
+		"author Author <author@example.com>\n"
+		"committer Committer <committer@example.com>\n"
+		"\n"
+		"Message";
+	assert_commit_fails(commit, 0);
+}
+
+void test_object_commit_parse__parsing_commit_without_tree_fails(void)
+{
+	const char *commit =
+		"author Author <author@example.com>\n"
+		"committer Committer <committer@example.com>\n"
+		"\n"
+		"Message";
+	assert_commit_fails(commit, 0);
+}
+
+void test_object_commit_parse__parsing_commit_without_author_fails(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8\n"
+		"committer Committer <committer@example.com>\n"
+		"\n"
+		"Message";
+	assert_commit_fails(commit, 0);
+}
+
+void test_object_commit_parse__parsing_commit_without_committer_fails(void)
+{
+	const char *commit =
+		"tree 3e7ac388cadacccdf1c6c5f3445895b71d9cb0f8\n"
+		"author Author <author@example.com>\n"
+		"\n"
+		"Message";
+	assert_commit_fails(commit, 0);
+}