This breaks a dependency cycle between GrDiffLine and GrDiffGroup. The
line only needed to need to know about groups, if it was of type
CONTEXT_CONTROL. But there was not really a need for the context control
to be modeled as a line. Moving this logic from line to group meant to
re-write the diff builders a little bit such that context control is now
handled a bit more explicitly, which is probably an improvement anyway.
This change is driven by the desire to convert GrDiffLine and
GrDiffGroup to TypeScript and the author did not see a great solution
for how to otherwise break the dependency cycle.
Change-Id: Id07b0b9b2c4f5caf7bcc99ac20164dd34c14653d
bcc6bb9eee