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
Colin Walters
63497c65f3