Make the defaultable fields defaultable

Also, add `git_commit_graph_writer_options_init`!

include/git2/sys/commit_graph.h

@@ -115,12 +115,13 @@ typedef struct {
 
 	/**
 	 * The number of commits in level N is less than X times the number of
-	 * commits in level N + 1.
+	 * commits in level N + 1. Default is 2.
 	 */
 	float size_multiple;
 
 	/**
 	 * The number of commits in level N + 1 is more than C commits.
+	 * Default is 64000.
 	 */
 	size_t max_commits;
 } git_commit_graph_writer_options;
@@ -129,7 +130,6 @@ typedef struct {
 #define GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT                                                       \
 	{                                                                                          \
 		GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION,                                           \
-		GIT_COMMIT_GRAPH_SPLIT_STRATEGY_SINGLE_FILE, 2.0f, 64000                           \
 	}
 
 /**

src/commit_graph.c

@@ -1143,6 +1143,18 @@ static int commit_graph_write_filebuf(const char *buf, size_t size, void *data)
 	return git_filebuf_write(f, buf, size);
 }
 
+int git_commit_graph_writer_options_init(
+	git_commit_graph_writer_options *opts,
+	unsigned int version)
+{
+	GIT_INIT_STRUCTURE_FROM_TEMPLATE(
+			opts,
+			version,
+			git_commit_graph_writer_options,
+			GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT);
+	return 0;
+}
+
 int git_commit_graph_writer_commit(
 		git_commit_graph_writer *w,
 		git_commit_graph_writer_options *opts)
@@ -1152,7 +1164,7 @@ int git_commit_graph_writer_commit(
 	git_buf commit_graph_path = GIT_BUF_INIT;
 	git_filebuf output = GIT_FILEBUF_INIT;
 
-	/* TODO: support options. */
+	/* TODO: support options and fill in defaults. */
 	GIT_UNUSED(opts);
 
 	error = git_buf_joinpath(

tests/core/structinit.c

 #include "clar_libgit2.h"
+#include <git2/sys/commit_graph.h>
 #include <git2/sys/config.h>
 #include <git2/sys/filter.h>
 #include <git2/sys/odb_backend.h>
@@ -97,6 +98,13 @@ void test_core_structinit__compare(void)
 		git_clone_options, GIT_CLONE_OPTIONS_VERSION, \
 		GIT_CLONE_OPTIONS_INIT, git_clone_options_init);
 
+	/* commit_graph_writer */
+	CHECK_MACRO_FUNC_INIT_EQUAL( \
+		git_commit_graph_writer_options, \
+		GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION, \
+		GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT, \
+		git_commit_graph_writer_options_init);
+
 	/* diff */
 	CHECK_MACRO_FUNC_INIT_EQUAL( \
 		git_diff_options, GIT_DIFF_OPTIONS_VERSION, \