The old logic for copying text in the diff viewer made some bad

assumptions that weren't valid in modern versions of Firefox, and could

end up with truncated text. It also didn't take into account

insert/delete lines, which needed to be ignored based on the side of the

diff being selected.
Part of the problem with the diff selection logic had to do with an

assumption that querying for <pre> tags would actually return the tags,

but that wasn't always the case depending on where the selection ended.
The new logic doesn't rely on the query we had before, but rather walks

the tree itself, filtering out insert/delete lines where appropriate.

The end result is that we can now populate the clipboard with the

entirety of the selected area, excluding the newlines from the

insert/delete lines.
This should be more future-proof and cross-browser than what we had

before.

Tested all sorts of selections on Chrome and Firefox:

1 entire line
Portions of a single line
Multi-line (entire lines)
Multi-line (starting/ending part-way through a line)
Selections starting/ending/spanning different marked up sections.