commit: filter out selinux label before commit

commit: filter out selinux label before commit

The new --selinux-policy added in [0] exposed a subtle issue in the way
we handle labeling during commit. The CI system in rpm-ostree hit this
when trying to make use of it[1].

Basically, because of the way we use a GVariant to represent xattrs, if
a file to be committed already has an SELinux label, the xattr object
ends up with *two* label entries. This of course throws off fsck later
on, since the checksum will have gone over both entries, even though the
on-disk file will only have a single label (in which the second entry
wins).

I confirmed that the `fsck` added in the installed test fails without
the rest of this patch.

[0] https://github.com/ostreedev/ostree/pull/1114
[1] https://github.com/projectatomic/rpm-ostree/pull/953

Closes: #1121
Approved by: cgwalters