Fix locking issues with git fetch --multiple --jobs=<n> and fetch.writeCommitGraph

The git fetch command recently learned to extend the --jobs=<n> option to cover the --multiple mode: it will run multiple fetches in parallel.

Together with the recent support to write commit-graphs automatically after each fetch by setting fetch.writeCommitGraph, this led to frequent issues where the commit-graph-chain.lock file could not be created because a parallel job had already created it.

This pair of patches first introduces the command-line option --write-commit-graph (together with the --no-* variant) and then uses it to avoid writing the commit-graph until all fetch jobs are complete.

I don't think that we will want to rush this into Git v2.24.0 because that release is imminent, and this is quite a corner case that I am fixing here. It's more of a FYI that I send this before v2.24.0 is available.

Johannes Schindelin (2):
fetch: add the command-line option --write-commit-graph
fetch: avoid locking issues between fetch.jobs/fetch.writeCommitGraph

 Documentation/fetch-options.txt |  4 ++++
 builtin/fetch.c                 | 10 ++++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

base-commit: efd54442381a2792186abc994060b8f7dd8b834b
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-443%2Fdscho%2Ffetch.writeCommitGraph-and-fetch-jobs-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-443/dscho/fetch.writeCommitGraph-and-fetch-jobs-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/443

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