commit-graph: add --split option to builtin

Add a new "--split" option to the 'git commit-graph write' subcommand. This
option allows the optional behavior of writing a commit-graph chain.

The current behavior will add a tip commit-graph containing any commits that
are not in the existing commit-graph or commit-graph chain. Later changes
will allow merging the chain and expiring out-dated files.

Add a new test script (t5324-split-commit-graph.sh) that demonstrates this
behavior.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 5 deletions

diff --git a/commit-graph.c b/commit-graph.c
index f0698b0599..1224309e5f 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1472,12 +1472,16 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 		}
 
 		if (ctx->base_graph_name) {
-			result = rename(ctx->base_graph_name,
-					ctx->commit_graph_filenames_after[ctx->num_commit_graphs_after - 2]);
+			const char *dest = ctx->commit_graph_filenames_after[
+						ctx->num_commit_graphs_after - 2];
 
-			if (result) {
-				error(_("failed to rename base commit-graph file"));
-				return -1;
+			if (strcmp(ctx->base_graph_name, dest)) {
+				result = rename(ctx->base_graph_name, dest);
+
+				if (result) {
+					error(_("failed to rename base commit-graph file"));
+					return -1;
+				}
 			}
 		} else {
 			char *graph_name = get_commit_graph_filename(ctx->obj_dir);