amend: abort if amending s0 with a different topic

This patch fixes issue of silently changing topic when amending on s0
(base of a topic stack) by aborting in that case. So now if you amend
s0 with a topic which is different from wdir parent it won't let you
amend until you update to the topic of wdir parent.

Changes in test file reflect the added behavior.
8 jobs for topic/default/fix-s0-amend-issue in 4 minutes and 47 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #156136
checks-py2

00:02:29

passed #156137
checks-py3

00:01:47

passed #156142
doc

00:00:36

manual #156135
allowed to fail manual
pytype
passed #156138
tests-py2-cext

00:04:47

passed #156139
tests-py2-pure

00:04:18

passed #156140
tests-py3-cext

00:04:19

passed #156141
tests-py3-pure

00:04:18