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, 13 insertions, 0 deletions

diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 139f7106f7..91ede622f0 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -570,6 +570,19 @@ test_expect_success 'LHS of refspec follows ref disambiguation rules' '
 	)
 '
 
+test_expect_success 'fetch.writeCommitGraph' '
+	git clone three write &&
+	(
+		cd three &&
+		test_commit new
+	) &&
+	(
+		cd write &&
+		git -c fetch.writeCommitGraph fetch origin &&
+		test_path_is_file .git/objects/info/commit-graphs/commit-graph-chain
+	)
+'
+
 # configured prune tests
 
 set_config_tristate () {