Introduce the GET_CONFIG IPC request

This introduces memory usage by one copy of the config file, which is an
acceptable trade-off for being able to easily revert data loss.
The default config is 6KB, user configs will be in the same ballpark.

fixes #2856

testcases/t/268-ipc-config.t

@@ -0,0 +1,55 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • http://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • http://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • http://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Verifies that the config file is returned raw via the IPC interface.
+# Ticket: #2856
+# Bug still in: 4.13-133-ge4da07e7
+use i3test i3_autostart => 0;
+use File::Temp qw(tempdir);
+
+my $tmpdir = tempdir(CLEANUP => 1);
+my $socketpath = $tmpdir . "/config.sock";
+ok(! -e $socketpath, "$socketpath does not exist yet");
+
+my $config = <<'EOT';
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+nop foo \
+continued
+
+set $var normal title
+for_window [title="$vartest"] border none
+EOT
+
+$config .= "ipc-socket $socketpath";
+
+my $pid = launch_with_config($config, dont_add_socket_path => 1, dont_create_temp_dir => 1);
+get_socket_path(0);
+my $i3 = i3(get_socket_path());
+$i3->connect->recv;
+
+my $cv = AE::cv;
+my $timer = AE::timer 0.5, 0, sub { $cv->send(0); };
+
+my $last_config = $i3->get_config()->recv;
+chomp($last_config->{config});
+is($last_config->{config}, $config,
+   'received config is not equal to written config');
+
+exit_gracefully($pid);
+
+done_testing;