Merge branch 'js/symlink-windows' into jch

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()`