dh_strip_nondeterminism: Deduplicate hardlinks via stat(2) to avoid issues...

dh_strip_nondeterminism: Deduplicate hardlinks via stat(2) to avoid issues when processing files in parallel. As strip-nondeterminism's handlers are not currently guaranteed to be atomic, one process can temporarily truncate a file which can cause errors in other threads processing the "same" file under a unique pathname. This was causing a FTBFS in packages that deduplicate hardlinks in their build process (eg. src:debian-handbook). (Closes: #922168)