Merge branch 'win32-accommodate-funny-drive-names'

While the only permitted drive letters for physical drives on Windows are letters of the US-English alphabet, this restriction does not apply to virtual drives assigned via `subst <letter>: <path>`. To prevent targeted attacks against systems where "funny" drive letters such as `1` or `!` are assigned, let's handle them as regular drive letters on Windows. This fixes CVE-2019-1351. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>