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.

We suggest to run `hg update .`, which will checkout the topic of 's0'
if it has one, otherwise clear the current topic.

Changes in test file reflect the added behavior.
8 jobs for topic/default/fix-s0-amend-issue in 4 minutes and 37 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed checks-py2 #235128

00:01:15

passed checks-py3 #235129

00:01:08

passed doc #235134

00:00:27

manual pytype #235127
allowed to fail manual
passed tests-py2-cext #235130

00:04:36

passed tests-py2-pure #235131

00:03:49

passed tests-py3-cext #235132

00:03:35

passed tests-py3-pure #235133

00:04:15