Support Docker-style whiteouts

This is to enable importing Docker layers as ostree commits, then
checking them out in a union.

The prototype work for this is in:
https://github.com/cgwalters/dlayer-ostree

Though it will likely ultimately end up in:
https://github.com/projectatomic/atomic

tests/basic-test.sh

@@ -387,6 +387,42 @@ assert_file_has_content test2-checkout/baz/cow moo
 assert_has_dir repo2/uncompressed-objects-cache
 echo "ok disable cache checkout"
 
+# Whiteouts
+cd ${test_tmpdir}
+mkdir -p overlay/baz/
+touch overlay/baz/.wh.cow
+touch overlay/.wh.deeper
+touch overlay/anewfile
+mkdir overlay/anewdir/
+touch overlay/anewdir/blah
+$OSTREE --repo=repo commit -b overlay -s 'overlay' --tree=dir=overlay
+rm overlay -rf
+
+for branch in test2 overlay; do
+    $OSTREE --repo=repo checkout --union --whiteouts ${branch} overlay-co
+done
+for f in .wh.deeper baz/cow baz/.wh.cow; do
+    assert_not_has_file overlay-co/${f}
+done
+assert_not_has_dir overlay-co/deeper
+assert_has_file overlay-co/anewdir/blah
+assert_has_file overlay-co/anewfile
+
+echo "ok whiteouts enabled"
+
+# Now double check whiteouts are not processed without --whiteouts 
+rm overlay-co -rf
+for branch in test2 overlay; do
+    $OSTREE --repo=repo checkout --union ${branch} overlay-co
+done
+for f in .wh.deeper baz/cow baz/.wh.cow; do
+    assert_has_file overlay-co/${f}
+done
+assert_not_has_dir overlay-co/deeper
+assert_has_file overlay-co/anewdir/blah
+assert_has_file overlay-co/anewfile
+echo "ok whiteouts disabled"
+
 cd ${test_tmpdir}
 rm -rf test2-checkout
 mkdir -p test2-checkout