properly handle symlinks as relative to the output dir

the code meant to make symlinks relative to the output directory, but
only actually succeeded at it for targets in the root of the output
directory; since we already check that the target is not breaking out of
the instance, we can generate the output symlink relative to itself and
fix the subdirred-symlink behavior

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

tests/instance/pages/subdir/relative-symlink-to-parent.md

@@ -0,0 +1 @@
+../more-metadata.md