ci (windows): transfer also the Git-tracked files to the test jobs

ci (windows): transfer also the Git-tracked files to the test jobs

Git's test suite is excruciatingly slow on Windows, mainly due to the
fact that it executes a lot of shell script code, and that's simply not
native to Windows.

To help with that, we established the pattern where the artifacts are
first built in one job, and then multiple test jobs run in parallel
using the artifacts built in the first job.

We take pains in transferring only the build outputs, and letting
`actions/checkout` fill in the rest of the files.

One major downside of that strategy is that the test jobs might fail to
check out the intended revision (e.g. because the branch has been
updated while the build was running, as is frequently the case with the
`seen` branch).

Let's transfer also the files tracked by Git, and skip the checkout step
in the test jobs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>