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 <johannes.schindelin@gmx.de>