checkout: add an extra checkout_overwrite mode

This is for issue projectatomic/rpm-ostree#365,
an extra option of overwrite mode is added to the checkout command
so that when there is "non-directory" file already exist
during checkout, the error will be handled.

Some tests are added for regression

Closes: #1116
Approved by: cgwalters

man/ostree-checkout.xml

@@ -97,6 +97,14 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--disjoint-union</option></term>
+
+                <listitem><para>
+                    When layering checkouts, error out if a file would be replaced, but add new files and directories
+                </para></listitem>
+            </varlistentry>
+
             <varlistentry>
                 <term><option>--allow-noent</option></term>