mingw: fix t5601-clone.sh

mingw: fix t5601-clone.sh

Since baaf233 (connect: improve check for plink to reduce false
positives, 2015-04-26), t5601 writes out a `plink.exe` for testing that
is actually a shell script. So the assumption that the `.exe` extension
implies that the file is *not* a shell script is now wrong.

Since there was no love for the idea of allowing `.exe` files to be
shell scripts on Windows, let's go the other way round: *make*
`plink.exe` a real `.exe`.

This fixes t5601-clone.sh in Git for Windows' SDK.

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