commit-graph: prefer default size_mult when given zero

In 50f26bd ("fetch: add fetch.writeCommitGraph config setting",
2019-09-02), the fetch builtin added the capability to write a
commit-graph using the "--split" feature. This feature creates
multiple commit-graph files, and those can merge based on a set
of "split options" including a size multiple. The default size
multiple is 2, which intends to provide a log_2 N depth of the
commit-graph chain where N is the number of commits.

However, I noticed during dogfooding that my commit-graph chains
were becoming quite large when left only to builds by 'git fetch'.
It turns out that in split_graph_merge_strategy(), we default the
size_mult variable to 2 except we override it with the context's
split_opts if they exist. In builtin/fetch.c, we create such a
split_opts, but do not populate it with values.

This problem is due to two failures:

 1. It is unclear that we can add the flag COMMIT_GRAPH_WRITE_SPLIT
    with a NULL split_opts.
 2. If we have a non-NULL split_opts, then we override the default
    values even if a zero value is given.

Correct both of these issues. First, do not override size_mult when
the options provide a zero value. Second, stop creating a split_opts
in the fetch builtin.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/commit-graph.c b/commit-graph.c
index 0aea7b2ae5..baa1a266b4 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1545,7 +1545,9 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
 
 	if (ctx->split_opts) {
 		max_commits = ctx->split_opts->max_commits;
-		size_mult = ctx->split_opts->size_multiple;
+
+		if (ctx->split_opts->size_multiple)
+			size_mult = ctx->split_opts->size_multiple;
 	}
 
 	g = ctx->r->objects->commit_graph;