Now that we have a `batch` mode, let's be explicit.
This is a follow-up to ce4786fc77 (mingw: change core.fsyncObjectFiles
= 1 by default, 2017-09-04) and will most likely have to be squashed
into it before upstreaming that patch (after the `batch` fsync mode was
upstreamed).
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>