Merge branch 'jk/empty-archive' into maint

"git archive" reports a failure when asked to create an archive out
of an empty tree.  It would be more intuitive to give an empty
archive back in such a case.

* jk/empty-archive:
  archive: handle commits with an empty tree
  test-lib: factor out $GIT_UNZIP setup

diff --cc t/t5003-archive-zip.sh
Simple merge

diff --cc t/test-lib.sh
Simple merge