Zero *both* buffers after seeking in a file.
Zero *both* buffers after seeking in a file.

Monotone-Parent: 785b7d2994ea79e7574a06f5a876150b75b25e4a
Monotone-Revision: 795d6677080b84d5134901986c87b9f486622192

FrancisRussell/lfmerge