Make sliding windows take open handles instead of files

This allows us to use the "-T" test on an open handle instead of a
path name.  Since the underlying "-T" has to read from the file, we
save at least one "open/close"-cycle of the file.

Signed-off-by: Niels Thykier <niels@thykier.net>
c/control-file: Flag directories as "not-a-file"

In the early days, there was sometimes a "root" dir entry and other
times not.  To work around that, the test also ignored dir entries.
However, now that the index never includes the "root" dir, the work
around is now obsolete.

Signed-off-by: Niels Thykier <niels@thykier.net>