mingw: fix quoting of arguments

mingw: fix quoting of arguments

We need to be careful to follow proper quoting rules. For example, if an
argument contains spaces, we have to quote them. Double-quotes need to
be escaped. Backslashes need to be escaped, but only if they are
followed by a double-quote character.

We need to be _extra_ careful to consider the case where an argument
ends in a backslash _and_ needs to be quoted: in this case, we append a
double-quote character, i.e. the backslash now has to be escaped!

The current code, however, fails to recognize that, and therefore can
turn an argument that ends in a single backslash into a quoted argument
that now ends in an escaped double-quote character. This allows
subsequent command-line parameters to be split and part of them being
mistaken for command-line options, e.g. through a maliciously-crafted
submodule URL during a recursive clone.

Technically, we would not need to quote _all_ arguments which end in a
backslash _unless_ the argument needs to be quoted anyway. For example,
`test\` would not need to be quoted, while `test \` would need to be.

To keep the code simple, however, and therefore easier to reason about
and ensure its correctness, we now _always_ quote an argument that ends
in a backslash.

This addresses CVE-2019-1350.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>