Added FollowLink() method to TreeEntry

[tf198]

Solution for #63.

As symlinks are all mode 120000 we have to traverse the tree which is more expensive than a straight check so I've implemented it as a FollowLink() method and the caller can determine the type

if entry.IsLink() {
  target, err = entry.FollowLink()
  // err: on broken symlinks and targets outside repo
  if target.IsDir() {
    // isDirectorySymlink action
  } else {
    // isFileSymlink action (or a link, but would need to detect loops in recursive calls)
  }
}

Have a patch ready for go-gitea/gitea#1416 if this is accepted...

For testing I had to add files directly to the tests/repo1_bare and include them in the commit. If there is a cleaner way to do this I'm happy to resubmit.

[tf198]

Fixed this warning but CI jumped the gun - trying to kick it again

[lunny]

LGTM