checkout: Add --allow-noent option

This is useful for the gnome-ostree build system where each build is
one commit, but it's split up into /runtime /devel /debug etc. trees.
Ideally we wouldn't have a /debug subdirectory for "noarch"
components for example.

So add an option to not error out if the given path doesn't exist in
the commit.

tests/t0000-basic.sh

@@ -19,7 +19,7 @@
 
 set -e
 
-echo "1..28"
+echo "1..30"
 
 . libtest.sh
 
@@ -208,3 +208,14 @@ rm -rf test2-checkout
 parent_rev_test2=$(ostree --repo=repo rev-parse test2)
 ${CMD_PREFIX} ostree --repo=shadow-repo checkout "${parent_rev_test2}" test2-checkout
 echo "ok checkout from shadow repo"
+
+cd ${test_tmpdir}
+rm -f expected-fail
+$OSTREE checkout test2 --subpath /enoent 2>/dev/null || touch expected-fail
+assert_has_file expected-fail
+echo "ok subdir enoent"
+
+cd ${test_tmpdir}
+$OSTREE checkout test2 --allow-noent --subpath /enoent 2>/dev/null
+echo "ok subdir noent"
+