border select. There was a fix before bmesh where it would require exactly two
vertices to be selected, but this was not ported over, and it also wasn't quite
correct.
This case should also work: click on two vertices, selected the path between
them, and then click on a 3rd vertex and select path, to extend the path further
from the 2nd to the 3rd vertex.
Now both use cases should work.
be4ac3a860