commit: Add _CONSUME modifier flag

For many cases of commit, we can actually optimize things by simply "adopting"
the object rather than writing a new copy. For example, in rpm-ostree package
layering.

We can only make that optimization though if we take ownership of the file. This
commit hence adds an API where a caller tells us to do so. For now, that just
means we `unlink()` the files/dirs as we go, but we can now later add the
"adopt" optimization.

Closes: #1255
Approved by: jlebon

bash/ostree

@@ -773,6 +773,7 @@ _ostree_commit() {
         --link-checkout-speedup
         --no-xattrs
         --orphan
+        --consume
         --skip-if-unchanged
         --table-output
         --tar-autocreate-parents