Some work was done for Review Board 2.5 to add line range indication to

text-only review e-mails, in order to give users who prefer plain text

e-mails some indication of where a comment was made. This work ended up

having some problems with the accuracy of the line ranges, the

usefulness of the ranges (it only showed the patched/modified ranges,

not those on the original file), the consistency (reply e-mails did not

show the same information), and reliability (it sometimes crashed).
This change completely redoes how these line ranges works, and does so

in a way where the line range calculations can be easily reused by other

parts of the codebase.
A new function, get_displayed_diff_line_ranges(), takes a list of

chunks and a virtual line range (indexes into a side-by-side diff table)

and returns a series of information about the human-readable displayed

ranges. The information covers the line numbers shown in the diff

viewer (limiting to compatible chunks where lines could occur), the

virtual line ranges that match those, and the beginning/ending chunks

for the range.
The template tag used for e-mail now uses this to more easily render

the line ranges. It's been updated to show both the original and patched

line ranges, where applicable. It also handles the case of a comment

that spans the entire file (something we document internally supporting,

but do not expose yet), which the old code would not be able to cope

with.
Last, but not least, the reply e-mails are now using the same template

tag to show a quoted response with the correct line information, instead

of the old behavior of showing a virtual line range.

Unit tests pass.
Verified the plain text e-mails for both reviews and replies against a

bunch of reviews I had on my dev server, testing various combinations of

chunks and ranges, and saw that the line numbers were matching up in all

cases.