Start the merging-rebase to v2.21.0-rc0

After cleaning up the branch thicket (in particular, update a few
branches to the versions that made it into core Git, and folding in the
fixup! and squash! commits), it is time to work on publishing a
pre-release: Git for Windows v2.21.0-rc0.

This commit starts the rebase of 27e3e78 to e4be047

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

Author: dscho