Correct new line numbers in diff viewer

With each new hunk, starting new line number was the same as starting old
line number. Parse diff line a bit further to get real new line number.

Fix HREFs for old line number links in both bootstrap3 and default themes
so that thay work.
Good point. Wikipedia [states](https://en.wikipedia.org/wiki/Diff#Unified_format) that
> In many versions of GNU diff, each range can omit the comma and trailing value _s_, in which case _s_ defaults to 1.

I'll fix this today, thanks.