Merge branch 'rs/archive-excluded-directory' into maint

"git archive" did not work well with pathspecs and the
export-ignore attribute.

We may want to resurrect the "we don't archive an empty directory"
bonus patch, but I do not mind merging the above early to 'next'
and leave it as a separate follow-up enhancement.
cf. <20170820090629.tumvqwzkromcykjf@sigill.intra.peff.net>

* rs/archive-excluded-directory:
  archive: don't queue excluded directories
  archive: factor out helper functions for handling attributes
  t5001: add tests for export-ignore attributes and exclude pathspecs