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

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