Start the merging-rebase to v2.22.0-rc1

This commit starts the rebase of b350563 to 556d02c

Note: this merge was performed on the `prepare-for-v2.22.0` branch that
tracks the v2.22.0-rc<N> updates, and from which Git for Windows'
previews are built. At this time, Git for Windows' `master` still
follows the v2.21.x train just in case that a release is needed before
v2.22.0.

But `master` advanced in the meantime, merging a PR to help with
commit-graph lock contention during `git gc` when `gc.writeCommitGraph`
is set to `true`.

So this is actually an octopus merge between v2.22.0-rc1,
`prepare-for-v2.22.0` *and* `master`, taking no changes from the latter
in preparation for rebuilding the (rebased) branch thicket of Git for
Windows on top of v2.22.0-rc1.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho