filter-branch: fix --prune-empty on parentless commits

filter-branch: fix --prune-empty on parentless commits

Previously, the git_commit_non_empty_tree function would always pass any
commit with no parents to git-commit-tree, regardless of whether the
tree was nonempty.  The new commit would then be recorded in the
filter-branch revision map, and subsequent commits which leave the tree
untouched would be correctly filtered.

With this change, parentless commits with an empty tree are correctly
pruned, and an empty file is recorded in the revision map, signifying
that it was rewritten to "no commits."  This works naturally with the
parent mapping for subsequent commits.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>