Merge branch 'fix-mingw-quoting-bug'

This patch fixes a vulnerability in the Windows-specific code where a
submodule names ending in a backslash were quoted incorrectly, and that
bug could be abused to insert command-line parameters e.g. to `ssh` in a
recursive clone.

Note: this bug is 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.

While at it, other quoting issues are fixed as well.

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