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.
Support hunk headers with no line counts

As Wikipedia states, "In many versions of GNU diff, each range can omit
the comma and trailing value s, in which case s defaults to 1".
Change regex to make line count optional (we don't use it anyway).
Merge pull request #481 from mikedld/diff-lines

Correct new line numbers in diff viewer