The issue was caused by non-continuous tangent space calculated for triangles.

This commit adds a Tangent input to Hair BSDF node which can be used to hook up
Tangent calculated form UV as an input to the node in order to make sure the
tangent space is continuous.

Doing this as an input instead of using default tangent layer from UV because of
several reasons:

• This way it's really easy to preserve compatibility with existing setups.

• Default UV map is not necessarily giving continuous space, one might want to use other tangent space sources or distort the space for some artistic decision.