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