When creating symlinks from git-style patches, make sure the symlinks don't
point above the current working directory. Otherwise, a subsequent patch could
use the symlink to write outside the working directory.
* src/pch.c (symlink_target_is_valid): New function to check for valid symlink
targets.
* src/util.c (move_file): Use symlink_target_is_valid() here.
* tests/symlinks: Add valid and invalid symlink test cases.
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