Previously, `Changes()` for a symlink to absolute path, with second-precision
mtime of the link itself always resulted in ENOENT.
This is because `compareFileContent()` was called for the link target path, without changing the root path.
Fixmoby/buildkit#172
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
