config: open configuration in commondir

A repository's configuartion file can always be found in the
GIT_COMMON_DIR, which has been newly introduced. For normal
repositories this does change nothing, but for working trees this
change allows to access the shared configuration file.

src/repository.c

@@ -862,8 +862,7 @@ static int load_config(
 	if ((error = git_config_new(&cfg)) < 0)
 		return error;
 
-	error = git_buf_joinpath(
-		&config_path, repo->path_repository, GIT_CONFIG_FILENAME_INREPO);
+	error = git_repository_item_path(&config_path, repo, GIT_REPOSITORY_ITEM_CONFIG);
 	if (error < 0)
 		goto on_error;
 

tests/worktree/config.c

+#include "clar_libgit2.h"
+#include "worktree_helpers.h"
+
+#define COMMON_REPO "testrepo"
+#define WORKTREE_REPO "testrepo-worktree"
+
+static worktree_fixture fixture =
+	WORKTREE_FIXTURE_INIT(COMMON_REPO, WORKTREE_REPO);
+
+void test_worktree_config__initialize(void)
+{
+	setup_fixture_worktree(&fixture);
+}
+
+void test_worktree_config__cleanup(void)
+{
+	cleanup_fixture_worktree(&fixture);
+}
+
+void test_worktree_config__open(void)
+{
+	git_config *cfg;
+
+	cl_git_pass(git_repository_config(&cfg, fixture.worktree));
+	cl_assert(cfg != NULL);
+
+	git_config_free(cfg);
+}
+
+void test_worktree_config__set(void)
+{
+	git_config *cfg;
+	int32_t val;
+
+	cl_git_pass(git_repository_config(&cfg, fixture.worktree));
+	cl_git_pass(git_config_set_int32(cfg, "core.dummy", 5));
+	git_config_free(cfg);
+
+	// reopen to verify configuration has been set in the
+	// common dir
+	cl_git_pass(git_repository_config(&cfg, fixture.repo));
+	cl_git_pass(git_config_get_int32(&val, cfg, "core.dummy"));
+	cl_assert_equal_i(val, 5);
+	git_config_free(cfg);
+}