Prevent rows of hyphens becoming dashes when back-porting to 2.1.

Prevent rows of hyphens becoming dashes when back-porting to 2.1.

Insert ligature breaks between hyphens where
they are separate in 2.2 but ligated by TeX in 2.1.

This fixes output with LyX 2.1 and prevents -- and --- becoming
en and em dash in the LyX source after a round-trip to 2.1
and back.

Solves lyx2lyx ctest failures with a warning about
possible line-break changes.