Start the merging-rebase to v2.26.0-rc0

The usual rebase with the usual skipped commits because they made it
upstream.

The only commit that was skipped for another reason is f21f3dc
(t1300: mark all test cases with funny filenames as !MINGW, 2017-07-03):
it is no longer necessary, as the respective tests no longer use funny
filenames on Windows.

Also just as usual: the newer PRs that can be upstreamed have been moved
from the tip toward the `ready-for-upstream` sub-thicket.

This commit starts the rebase of f3f9031 to fedb91a

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

Author: dscho