repo: Add a checkout option to not hardlink zero-sized files

In rpm-ostree we've hit a few cases where hardlinking zero-sized
files causes us problems.  The most prominent is lock files in
`/usr/etc`, such as `/usr/etc/selinux/semanage.LOCK`.  If there
are two zero-sized lock files to grab, but they're hardlinked,
then locking will fail.

Another case here is if one is using ostree inside a container
and don't have access to FUSE (i.e. `rofiles-fuse`), then the
ostree hardlinking can cause files that aren't ordinarily hardlinked
to become so, and mutation of one mutates all.  An example where
this is concerning is Python `__init__.py` files.

Now, these lock files should clearly not be in the tree to begin
with, but - we're not gaining a huge amount by hardlinking these
files either, so let's add an option to disable it.

Closes: #1752
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -933,7 +933,8 @@ typedef struct {
   gboolean no_copy_fallback;
   gboolean force_copy; /* Since: 2017.6 */
   gboolean bareuseronly_dirs; /* Since: 2017.7 */
-  gboolean unused_bools[5];
+  gboolean force_copy_zerosized; /* Since: 2018.9 */
+  gboolean unused_bools[4];
   /* 4 byte hole on 64 bit */
 
   const char *subpath;