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, 8 insertions, 0 deletions

diff --git a/Documentation/config/feature.txt b/Documentation/config/feature.txt
index 545522f306..875f8c8a66 100644
--- a/Documentation/config/feature.txt
+++ b/Documentation/config/feature.txt
@@ -17,6 +17,14 @@ which can improve `git push` performance in repos with many files.
 +
 * `fetch.negotiationAlgorithm=skipping` may improve fetch negotiation times by
 skipping more commits at a time, reducing the number of round trips.
++
+* `fetch.writeCommitGraph=true` writes a commit-graph after every `git fetch`
+command that downloads a pack-file from a remote. Using the `--split` option,
+most executions will create a very small commit-graph file on top of the
+existing commit-graph file(s). Occasionally, these files will merge and the
+write may take longer. Having an updated commit-graph file helps performance
+of many Git commands, including `git merge-base`, `git push -f`, and
+`git log --graph`.
 
 feature.manyFiles::
 	Enable config options that optimize for repos with many files in the