[PATCH 0/2] fix git-archive with empty trees

I noticed that "git archive" will barf when the root tree is empty. Instead, it should probably return an empty archive. I doubt many people really care about this corner case in practice, but it seems like we should handle it more gracefully (and it's an easy fix).

It came to my attention because we track failed git-archive invocations at GitHub, and a particular repo had a two commits: adding some content, then reverting the original commit. You can also get there with "commit --allow-empty" on a new branch (which is what the tests do).

I didn't bother even looking at empty subtrees. AFAIK, git should never produce them (it omits the tree entirely if there is no content in it). You would have to fake it using hash-object manually. I suspect it would work just fine, as we already exercise the empty-dir code paths in the tests I did add.

  [1/2]: test-lib: factor out $GIT_UNZIP setup
  [2/2]: archive: handle commits with an empty tree

-Peff