bin/commit: Fix --tree=tar with --selinux-policy

The logic for `--selinux-policy` ended up in the `--tree=dir`
path, but there's no reason for that.  Fix the imported
labeling with `--tree=tar`.  Prep for use with containers.

We had this bug because the previous logic was trying to avoid
duplicating the code for generic `--selinux-policy` and
the case of `--selinux-policy-from-base --tree=dir`.

It's a bit more code, but it's cleaner if we dis-entangle them.

tests/kolainst/destructive/itest-label-selinux.sh

@@ -104,3 +104,16 @@ assert_file_has_content newls.txt ':lib_t:'
 ostree ls -X newbase /usr/etc/some.conf > newls.txt
 assert_file_has_content newls.txt ':etc_t:'
 echo "ok commit --selinux-policy-from-base"
+
+rm rootfs -rf
+mkdir rootfs
+mkdir -p rootfs/usr/{bin,lib,etc}
+echo 'somebinary' > rootfs/usr/bin/somebinary
+ls -Z rootfs/usr/bin/somebinary > lsz.txt
+assert_not_file_has_content lsz.txt ':bin_t:'
+rm -f lsz.txt
+tar -C rootfs -cf rootfs.tar .
+ostree commit -b newbase --selinux-policy / --tree=tar=rootfs.tar
+ostree ls -X newbase /usr/bin/somebinary > newls.txt
+assert_file_has_content newls.txt ':bin_t:'
+echo "ok commit --selinux-policy with --tree=tar"