checkout: Merge union/add logic for copies during checkout

We really have an astonishing variety of similar functions which write files and
symlinks. I was working on a different PR and the duplication between the
union-mode and add-mode/none-mode checkout functions bothered me.

I realized that the "handle EEXIST" tri-state maps directly to the
`GLnxLinkTmpfileReplaceMode`, so deduping things makes even more sense.

Closes: #801
Approved by: jlebon

tests/test-basic-root.sh

@@ -0,0 +1,56 @@
+#!/bin/bash
+#
+# Copyright (C) 2011 Colin Walters <walters@verbum.org>
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the
+# Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+# Boston, MA 02111-1307, USA.
+
+set -euo pipefail
+
+. $(dirname $0)/libtest.sh
+
+id=$(id -u)
+
+if test ${id} != 0; then
+    skip "continued basic tests must be run as root (possibly in a container)"
+fi
+
+setup_test_repository "bare"
+
+echo "1..1"
+
+nextid=$(($id + 1))
+
+rm checkout-test2 -rf
+$OSTREE checkout test2 checkout-test2
+$OSTREE commit ${COMMIT_ARGS} -b test2 --tree=ref=test2 --owner-uid=$nextid
+$OSTREE ls test2 baz/cow > ls.txt
+assert_file_has_content ls.txt '-00644 '${nextid}' '${id}
+# As bare and running as root (e.g. Docker container), do some ownership tests
+# https://github.com/ostreedev/ostree/pull/801
+# Both hardlinks and copies should respect ownership, but we don't have -C yet;
+# add it when we do.
+for opt in -H; do
+    rm test2-co -rf
+    $OSTREE checkout ${opt} test2 test2-co
+    assert_streq "$(stat -c '%u' test2-co/baz/cow)" ${nextid}
+    assert_streq "$(stat -c '%u' test2-co/baz/alink)" ${nextid}
+done
+rm test2-co -rf
+# But user mode doesn't
+$OSTREE checkout -U test2 test2-co
+assert_streq "$(stat -c '%u' test2-co/baz/cow)" ${id}
+assert_streq "$(stat -c '%u' test2-co/baz/alink)" ${id}
+echo "ok ownership"