mingw: support spawning programs containing spaces in their names

mingw: support spawning programs containing spaces in their names

On some older Windows versions (e.g. Windows 7), the CreateProcessW()
function does not really support spaces in its first argument,
lpApplicationName. But it supports passing NULL as lpApplicationName,
which makes it figure out the application from the (possibly quoted)
first argument of lpCommandLine.

Let's use that trick (if we are certain that the first argument matches
the executable's path) to support launching programs whose path contains
spaces.

We will abuse the test-fake-ssh.exe helper to verify that this works and
does not regress.

This fixes https://github.com/git-for-windows/git/issues/692

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