Merge branch 'am/sparse-index-name-hash-fix'

Avoid adding directory path to a sparse-index tree entries to the
name-hash, since they would bloat the hashtable without anybody
querying for them.  This was done already for a single threaded
part of the code, but now the multi-threaded code also does the
same.

* am/sparse-index-name-hash-fix:
  name-hash: don't add sparse directories in threaded lazy init