Remove unowned files from /mingw32 and /mingw64

This repeats the exercise from git-for-windows#35, but
excludes a few files that might actually be relevant:

This trick was performed by:

  f() {
    comm -23 \
      <(git -C / ls-files $1\* |
        sort) \
      <(git -C /var/lib/pacman/local/ grep ^$1 \*/files |
        sed -n 's,^.*:\(.*[^/]\)$,\1,p' |
        sort)
  }
  f mingw |
  xargs -d '\n' -r git -C / rm

  git restore --staged mingw32/etc/git* mingw64/etc/git* mingw64/share

Signed-off-by: Jeremy Drake <[email protected]>