Start the merging-rebase to v2.21.0-rc2

This is a "dummy" rebase, because we already rebased to that tag.

So why rebase, then? For multiple reasons:

1. to pull in `origin/master`'s commit history (without actually taking
   its changes, because we already forward-ported the
   dont-spawn-gzip-in-archive patches to `rebase-to-v2.21.0`.

2. to reorder some built-in stash related patches into the correct spot.

3. to move the dont-spawn-gzip-in-archive patches into the
   ready-for-upstream branch thicket.

This commit starts the rebase of 948d034 to 1c668b7

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho