Scroll diff with arrow keys in log view

This commit introduces the VIEW_NO_PARENT_NAV flag and adds it to the
log view. This allows the scrolling commands to fall through from the
pager to the diff when the diff is viewed in the log mode.

Signed-Off-By: Kumar Appaiah <a.kumar@alumni.iitm.ac.in>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>