MT#64357 Ignore /etc/localtime

Our cowbuilder based chroots that are also used for piuparts, provide
/etc/localtime by default (seems to get created by mmdebstrap):

  % ls -la /var/cache/pbuilder.persistent/base-trixie-amd64.cow/etc/localtime
  lrwxrwxrwx 1 root root 23 Feb  2 18:24 /var/cache/pbuilder.persistent/base-trixie-amd64.cow/etc/localtime -> /usr/share/zoneinfo/UTC

Now as of tzdata v2017a-1, tzdata removes file /etc/localtime on
purge, see https://bugs.debian.org/854141

Until piuparts also ignores this file change (known as
https://bugs.debian.org/1080295), let's make sure we don't report this
as a problem, by ignoring it within our piuparts wrapper.

Fixes:

| ERROR: FAIL: After purging files have disappeared:
| /etc/localtime -> /usr/share/zoneinfo/UTC not owned

FTR:

% piuparts --help 2>&1 | grep -A3 -- ' -i '
  -i FILENAME, --ignore=FILENAME
                        Add FILENAME to list of filenames to be ignored when
                        comparing changes to chroot.FILENAMES prefixed with
                        ':' will be reported verbosely.

Change-Id: Ie1c1e0246b7540a7806aa94fc9338fd3a1e56457

wrapper/piuparts_runner

@@ -66,7 +66,8 @@ piuparts_cmdline="sudo piuparts \
   --warn-on-leftovers-after-purge --skip-logrotatefiles-test --log-file=piuparts.txt \
   --distribution=$distribution --mirror=$mirror --keep-sources-list \
   --scriptsdir=$SCRIPTSDIR --tmpdir=$PIUPARTS_TMPDIR \
-  --existing-chroot=$PIUPARTS_BASEDIR ${UPDATE_RETRIES:-}"
+  --existing-chroot=$PIUPARTS_BASEDIR ${UPDATE_RETRIES:-} \
+  -i /etc/localtime"
 echo "*** Using '$piuparts_cmdline' as piuparts_cmdline ***"
 
 # wrapper for CE vs PRO vs CARRIER packages