Absolute target in symlinks lead to dirty repo

[sttts]

Commit a symlink with an absolute path (e.g./usr/bin/vim) to a repo. The worktree is always considered dirty.

This is why:

• go-git uses go-billy's chroot filesystem wrapper to abstract the worktree.
• go-billy rewrites the symlink target path in symlink.ReadLink https://github.com/src-d/go-billy/blob/83cf655d40b15b427014d7875d10850f96edba14/helper/chroot/chroot.go#L195 (it is unclear why the logic in there is as it is: why should an absolute path be made relative and then prefixed with /?)
• go-git computes the symlink file hash over the returned target path. This path is different after upper mangling than the symlink target path hash of the index.
• go-git thinks the worktree is dirty.