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

man/ostree-commit.xml

@@ -167,6 +167,17 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--consume</option></term>
+
+                <listitem><para>
+                  When committing from a local directory (i.e. not an archive or --tree=ref),
+                  assume ownership of the content.  This may simply involve deleting it,
+                  but if possible, the content may simply be <literal>rename()</literal>ed
+                  into the repository rather than creating a new copy.
+                </para></listitem>
+            </varlistentry>
+
             <varlistentry>
                 <term><option>--statoverride</option>="PATH"</term>