Can't clean broken symbolic links

[duanyao]

Steps to reproduce:

• ln -s nonexists alink . nonexists is a non-existing file.
• Create a clean task cleanBrokenLink: ['alink'] and run it. The output should be:

  Running "clean:cleanBrokenLink" (clean) task
  >> 1 path cleaned.
  

• ls -l alink output: alink -> nonexists, showing that alink is not cleaned.

Note that broken symlinks could show up easily when cleaning a dir containing symlinks: if a link target was removed before the link is tried to remove, the link becomes broken.

Root cause: clean() in tasks/clean.js tests a filepath with grunt.file.exists() before trying to remove it, but grunt.file.exists() return false for a broken symbolic link (because of fs.exists() it calling), so clean() skips to remove it.

Possible solution: replace grunt.file.exists() with a fs.lstatSync() based solution, because it doesn't follow links. Additionally, grunt.file.isPathInCwd() also suffers from broken-symbolic-link problem.