checkout/commit: Use glnx_regfile_copy_bytes() if possible

Rather than `g_output_stream_splice()`, where the input is a regular
file.

See https://github.com/GNOME/libglnx/pull/44 for some more information.

I didn't try to measure the performance difference, but seeing the
read()/write() to/from userspace mixed in with the pointless `poll()` annoyed me
when reading strace.

As a bonus, we will again start using reflinks (if available) for `/etc`,
which is a regression from the https://github.com/ostreedev/ostree/pull/797
changes (which before used `glnx_file_copy_at()`).

Also, for the first time we'll use reflinks when doing commits from file-backed
content. This happens in `rpm-ostree compose tree` today for example.

Update submodule: libglnx

Closes: #817
Approved by: jlebon

tests/ci-commitmessage-submodules.sh

@@ -40,6 +40,7 @@ git log --pretty=oneline origin/master.. | while read logline; do
             echo "Commit $commit modifies submodule: $submodule"
             expected_match="Update submodule: $submodule"
             if ! grep -q -e "$expected_match" ${tmpd}/log.txt; then
+                sed -e 's,^,# ,' < ${tmpd}/log.txt
                 echo "error: Commit message for ${commit} changes a submodule, but does not match regex ${expected_match}"
                 exit 1
             fi