Adding a "are you sure?" prompt when user try to get diff between commits that is too big

Prompt "are you sure?" when user try to get diff between commits that is over 100 lines.

Perhaps a better location for "are you sure?" check is at the textdiff.js and not at the commitlinediff.js but since textdiff.js doesn't have number of lines changed info this is little difficult.  `git status` doesn't provide a number of changed lines which is disappointing.  

(100 is an arbitrary number but it doesn't really make sense to see more then 100 lines of  diffs in tiny window.  50 maybe even a better number.)