Merge branch 'fix-msys2-quoting-bugs'

These patches fix several bugs in quoting arguments when spawning shell
scripts on Windows.

Note: these bugs are Windows-only, as we have to construct a command
line for the process-to-spawn, unlike Linux/macOS, where `execv()`
accepts an already-split command line.

Furthermore, these fixes were not included in the CVE-2019-1350 part of
v2.14.6 because the Windows-specific quoting when spawning shell scripts
was contributed from Git for Windows into Git only in the v2.21.x era.

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