Upstream symbolic link support on Windows from Git-for-Windows.
* js/symlink-windows:
mingw: special-case index entries for symlinks with buggy size
mingw: emulate `stat()` a little more faithfully
mingw: try to create symlinks without elevated permissions
mingw: add support for symlinks to directories
mingw: implement basic `symlink()` functionality (file symlinks only)
mingw: implement `readlink()`
mingw: allow `mingw_chdir()` to change to symlink-resolved directories
mingw: support renaming symlinks
mingw: handle symlinks to directories in `mingw_unlink()`
mingw: add symlink-specific error codes
mingw: change default of `core.symlinks` to false
mingw: factor out the retry logic
mingw: compute the correct size for symlinks in `mingw_lstat()`
mingw: teach dirent about symlinks
mingw: let `mingw_lstat()` error early upon problems with reparse points
mingw: drop the separate `do_lstat()` function
mingw: implement `stat()` with symlink support
mingw: don't call `GetFileAttributes()` twice in `mingw_lstat()`
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