topic: check that repo.currenttopic is set before using it as an argument
repo.currenttopic can be an empty string, which means there's no current topic
set. This fact can be used to short-circuit some checks.
Also, this avoids passing an empty string as an argument to topic() revset.
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"
marked working directory as topic: testing-shelve
$ hg topic
* testing-shelve (0 changesets)
$ hg ci -m "First commit" -A
adding a
active topic 'testing-shelve' grew its first changeset
(see 'hg help topics' for more information)
$ hg topic
* testing-shelve (1 changesets)
$ echo " World" >> a
$ hg stack
### topic: testing-shelve
### target: default (branch)
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 (1 changesets)
$ hg stack
### topic: testing-shelve
### target: default (branch)
t1@ First commit (current)
unshelve test
-------------
$ hg unshelve
unshelving change 'default'
$ hg topic
* testing-shelve (1 changesets)
$ hg stack
### topic: testing-shelve
### target: default (branch)
t1@ First commit (current)