Add subroutine to display one-line summary of hunks

Add subroutine to display one-line summary of hunks

This commit implements a rather simple-minded mechanism to display a
one-line summary of the hunks in an array ref.  The display consists of
the line numbers and the first changed line, truncated to 80 characters.
20 lines are displayed at a time, and the index of the first undisplayed
line is returned, allowing the caller to display more if desired.  (The 20
and 80 should be made configurable.)

Signed-off-by: William Pursell <bill.pursell@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>