ostree commit: Fix combining trees with multiple --tree=ref arguments

You'd expect

    ostree commit --tree=ref=A --tree=ref=B

to produce a commit with the union of the trees given.  Instead you'd get
a commit with the contents of just the latter commit.  This was due to an
optimisation where we'd skip filling out the `files` and `subdirs`
members of the mtree, just filling in the metadata instead.  This backfires
becuase this same code relies on checking the `files` and `subdirs` members
itself to work out whether the mtree is empty.

This commit removes the optimisation, fixing the bug.  Maybe there's a way
to keep the optimisation and still fix the bug but it's not obvious to
me.

Closes: #581
Approved by: cgwalters

tests/basic-test.sh

@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..59"
+echo "1..60"
 
 $OSTREE checkout test2 checkout-test2
 echo "ok checkout"
@@ -194,6 +194,20 @@ cd ${test_tmpdir}/checkout-test2-4
 $OSTREE commit -b test2 -s "no xattrs" --no-xattrs
 echo "ok commit with no xattrs"
 
+mkdir tree-A tree-B
+touch tree-A/file-a tree-B/file-b
+
+$OSTREE commit -b test3-1 -s "Initial tree" --tree=dir=tree-A
+$OSTREE commit -b test3-2 -s "Replacement tree" --tree=dir=tree-B
+$OSTREE commit -b test3-combined -s "combined tree" --tree=ref=test3-1 --tree=ref=test3-2
+
+$OSTREE checkout test3-combined checkout-test3-combined
+
+assert_has_file checkout-test3-combined/file-a
+assert_has_file checkout-test3-combined/file-b
+
+echo "ok commit combined ref trees"
+
 # NB: The + is optional, but we need to make sure we support it
 cd ${test_tmpdir}
 cat > test-statoverride.txt <<EOF