Merge branch 'prevent-name-squatting-on-windows'

This patch series fixes an issue where Git could formerly have been
tricked into creating a `.git` file with an unexpected (and therefore
unprotected) NTFS short name.

Incidentally, it also fixes an issue where a tree entry containing a
backslash could be tricked into following a symbolic link, i.e. Git
could be tricked into writing files outside the worktree.

Signed-off-by: Johannes Schindelin <redacted>