fetch: add fetch.writeCommitGraph config setting

The commit-graph feature is now on by default, and is being
written during 'git gc' by default. Typically, Git only writes
a commit-graph when a 'git gc --auto' command passes the gc.auto
setting to actualy do work. This means that a commit-graph will
typically fall behind the commits that are being used every day.

To stay updated with the latest commits, add a step to 'git
fetch' to write a commit-graph after fetching new objects. The
fetch.writeCommitGraph config setting enables writing a split
commit-graph, so on average the cost of writing this file is
very small. Occasionally, the commit-graph chain will collapse
to a single level, and this could be slow for very large repos.

For additional use, adjust the default to be true when
feature.experimental is enabled.

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

1 files changed, 4 insertions, 0 deletions

diff --git a/repo-settings.c b/repo-settings.c
index 3779b85c17..05546db98e 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -49,10 +49,14 @@ void prepare_repo_settings(struct repository *r)
 		UPDATE_DEFAULT_BOOL(r->settings.index_version, 4);
 		UPDATE_DEFAULT_BOOL(r->settings.core_untracked_cache, UNTRACKED_CACHE_WRITE);
 	}
+	if (!repo_config_get_bool(r, "fetch.writecommitgraph", &value))
+		r->settings.fetch_write_commit_graph = value;
 	if (!repo_config_get_bool(r, "feature.experimental", &value) && value) {
 		UPDATE_DEFAULT_BOOL(r->settings.pack_use_sparse, 1);
 		UPDATE_DEFAULT_BOOL(r->settings.fetch_negotiation_algorithm, FETCH_NEGOTIATION_SKIPPING);
+		UPDATE_DEFAULT_BOOL(r->settings.fetch_write_commit_graph, 1);
 	}
+	UPDATE_DEFAULT_BOOL(r->settings.fetch_write_commit_graph, 0);
 
 	/* Hack for test programs like test-dump-untracked-cache */
 	if (ignore_untracked_cache_config)