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

tests/basic-test.sh

@@ -21,7 +21,7 @@
 
 set -euo pipefail
 
-echo "1..$((83 + ${extra_basic_tests:-0}))"
+echo "1..$((84 + ${extra_basic_tests:-0}))"
 
 CHECKOUT_U_ARG=""
 CHECKOUT_H_ARGS="-H"
@@ -694,6 +694,21 @@ for v in bin link; do
 done
 echo "ok checkout union identical conflicts"
 
+cd ${test_tmpdir}
+rm files -rf && mkdir files
+touch files/anemptyfile
+touch files/anotheremptyfile
+$CMD_PREFIX ostree --repo=repo commit --consume -b tree-with-empty-files --tree=dir=files
+$CMD_PREFIX ostree --repo=repo checkout ${CHECKOUT_H_ARGS} -z tree-with-empty-files tree-with-empty-files
+if files_are_hardlinked tree-with-empty-files/an{,other}emptyfile; then
+    fatal "--force-copy-zerosized failed"
+fi
+rm tree-with-empty-files -rf
+$CMD_PREFIX ostree --repo=repo checkout ${CHECKOUT_H_ARGS} tree-with-empty-files tree-with-empty-files
+assert_files_hardlinked tree-with-empty-files/an{,other}emptyfile
+rm tree-with-empty-files -rf
+echo "ok checkout --force-copy-zerosized"
+
 cd ${test_tmpdir}
 rm files -rf && mkdir files
 mkdir files/worldwritable-dir