topic: provide more information when behind count cannot be computed
The behind count has been confusing for various user and the associated error
message is even worth. We try to clarify it to see if that helps.
testing topic with shelve extension
------------------------------------
$ . "$TESTDIR/testlib/topic_setup.sh"
$ hg init repo
$ cd repo
$ cat <<EOF >>.hg/hgrc
> [extensions]
> shelve=
> EOF
$ touch a
$ echo "Hello" >> a
$ hg topic "testing-shelve"
$ hg topic
* testing-shelve
$ hg ci -m "First commit" -A
adding a
$ hg topic
* testing-shelve
$ echo " World" >> a
$ hg stack
### topic: testing-shelve
### branch: default
t1@ First commit (current)
shelve test
-----------
$ hg shelve
shelved as default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg topic
* testing-shelve
$ hg stack
### topic: testing-shelve
### branch: default
t1@ First commit (current)
unshelve test
-------------
$ hg unshelve
unshelving change 'default'
$ hg topic
* testing-shelve
$ hg stack
### topic: testing-shelve
### branch: default
t1@ First commit (current)