When shell scripts access a $TMPDIR variable containing backslashes,
they will be mistaken for escape characters. Let's not let that happen
by converting them to forward slashes.
This partially fixes t7800 with MSYS2.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>