One way to do it is to pull master into the topic branch:
------------
- $ git checkout topic
+ $ git switch topic
$ git merge master
o---*---o---+ topic
in which case the final commit graph would look like this:
------------
- $ git checkout topic
+ $ git switch topic
$ git merge master
$ ... work on both topic and master branches
- $ git checkout master
+ $ git switch master
$ git merge topic
o---*---o---+---o---o topic
top of the tip before the test merge:
------------
- $ git checkout topic
+ $ git switch topic
$ git merge master
$ git reset --hard HEAD^ ;# rewind the test merge
$ ... work on both topic and master branches
- $ git checkout master
+ $ git switch master
$ git merge topic
o---*---o-------o---o topic