hg pick with no active topic and with a different active topic (issue6406)
https://bz.mercurial-scm.org/show_bug.cgi?id=6406
For prior discussions on this behavior see also
https://foss.heptapod.net/mercurial/evolve/-/merge_requests/313
https://foss.heptapod.net/mercurial/evolve/-/merge_requests/390

  $ . "$TESTDIR/testlib/common.sh"

  $ cat << EOF >> "$HGRCPATH"
  > [phases]
  > publish = no
  > [extensions]
  > evolve =
  > topic =
  > EOF

#testcases inmemory ondisk
#if inmemory
  $ cat >> $HGRCPATH <<EOF
  > [experimental]
  > evolution.in-memory = yes
  > EOF
#endif

  $ hg init issue6406
  $ cd issue6406

  $ mkcommit ROOT

  $ hg debug-topic-namespace aaa
  marked working directory as topic namespace: aaa
  $ hg topic a-things
  marked working directory as topic: a-things
  $ mkcommit apple
  active topic 'a-things' grew its first changeset
  (see 'hg help topics' for more information)

  $ hg up 'desc("ROOT")'
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ hg debug-topic-namespace bbb
  marked working directory as topic namespace: bbb
  $ hg topic b-things
  marked working directory as topic: b-things
  $ mkcommit banana
  active topic 'b-things' grew its first changeset
  (see 'hg help topics' for more information)
  $ mkcommit blackberry

  $ hg up 'desc("apple")'
  switching to topic-namespace aaa
  switching to topic a-things
  1 files updated, 0 files merged, 2 files removed, 0 files unresolved

This is what the help text says about this issue

  $ hg help pick | grep 'active topic'
      The resulting changeset will have the current active topic. If there's no
      active topic set, the resulting changeset will also not have any topic.

wdir has no active topic: pick should clear topic of the resulting cset

  $ hg debug-topic-namespace --clear
  $ hg topic --clear
  $ hg pick 'desc("banana")'
  picking 2:fcda3d8dafd2 "banana"
  1 new orphan changesets
  $ hg log -r . -T '{rev}: {desc} ({fqbn})\n'
  4: banana (default)
  $ hg debug-topic-namespace
  none
  $ hg topic --current
  no active topic
  [1]

wdir has active topic: pick should use the active topic for the resulting cset

  $ hg debug-topic-namespace everything
  marked working directory as topic namespace: everything
  $ hg topic all-things
  marked working directory as topic: all-things
  $ hg pick 'desc("blackberry")'
  picking 3:48bbfbece8fa "blackberry"
  active topic 'all-things' grew its first changeset
  (see 'hg help topics' for more information)
  $ hg log -r . -T '{rev}: {desc} ({fqbn})\n'
  5: blackberry (default//everything/all-things)
  $ hg debug-topic-namespace
  everything
  $ hg topic --current
  all-things

  $ hg log -GT '{rev}: {desc} ({fqbn})\n{join(extras, " ")}\n\n'
  @  5: blackberry (default//everything/all-things)
  |  branch=default topic=all-things topic-namespace=everything
  |
  o  4: banana (default)
  |  branch=default
  |
  o  1: apple (default//aaa/a-things)
  |  branch=default topic=a-things topic-namespace=aaa
  |
  o  0: ROOT (default)
     branch=default