Merge branch 'jk/empty-archive'

"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

t/test-lib.sh

@@ -760,3 +760,9 @@ test_lazy_prereq AUTOIDENT '
 # When the tests are run as root, permission tests will report that
 # things are writable when they shouldn't be.
 test -w / || test_set_prereq SANITY
+
+GIT_UNZIP=${GIT_UNZIP:-unzip}
+test_lazy_prereq UNZIP '
+	"$GIT_UNZIP" -v
+	test $? -ne 127
+'