stop symlink check once the top of the working tree is reached

stop symlink check once the top of the working tree is reached

Avoid complaining that a file with "is beyond a symbolic link" when the
filepath is absolute and the symlink in question is not actually inside the
git repository.

This assumes that inodes remain stable while the command is running.
I think they always will, the filesystems where they are unstable change
them across mounts. (If inodes were not stable, it would just complain about
symlinks in the path that are not inside the working tree.)

(On windows, I don't want to assume anything about inodes, they could be
random numbers for all I know. But if they were, this would still be ok, as
long as windows doesn't have symlinks that are detected by isSymbolicLink.
Which seems a fair bet.)