Start the merging-rebase to v2.28.0

As a last step before v2.29.0-rc0, let's clean up Git for Windows'
branch thicket a bit, in particular reintroducing the
`ready-for-upstream` sub-thicket, and replacing the scattered FSCache
topics with a somewhat consolidated version.

This commit starts the rebase of e62b20a to 4c8bcdd and does not
introduce any changes relative to 47ae905.

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

Author: dscho