topics: consistently use empty string instead of None
This agrees with repo.currenttopic which uses ''. A subsequent patch
is going to provide a context topic method that returns the empty
string.
#!/bin/sh
# This file holds logic that is used in many tests.
# It can be called in a test like this:
# $ . "$TESTDIR/testlib"
# Enable obsolete markers and enable extensions
cat >> $HGRCPATH << EOF
[experimental]
evolution=createmarkers,exchange
[extensions]
rebase=
EOF
echo "topic=$(echo $(dirname $TESTDIR))/src/topic" >> $HGRCPATH