Improve handling of large diffs
Diffs with a large number of changed lines time out (504 HTTP error) or
generate a HTML page that's so heavy web browsers struggle with it.
https://github.com/gitlabhq/gitlabhq/pull/5014 introduced limits on
commit line count so that only a safe portion is rendered. This was
later undone by code refactoring in be5b6db8, e0eb4803 and c741fcab.
This patch re-introduces a safe limit on number of lines.
See merge request !539
