When generating an interdiff, we need to match up files from the lists

of filediffs on both sides of an interdiff range. We used to do this by

comparing the source filenames, which worked in the common cases.

However, this didn't account for diffs that did something more complex,

like modify a file in diff 1 and then delete + re-introduce in diff 2,

or copy and modify twice, or other such operations. These were a lot

more rare, but could happen, particularly with Subversion.
This new logic works a lot harder to find the appropriate filediffs to

match, handling both simple and pathological cases. It starts out by

attempting to match all filediffs that have the same source filenames,

destination filenames, and new/deleted flags.
After that finishes, any remaining filediffs and interdiffs are matched

based on the source filename. A single source file on diff 1 can match

up with several on diff 2, handling the case where a file is copied to a

couple different places and modified. Any filediffs on diff 1 that fail

to match filediffs on diff 2 are shown as reverted.
Finally, any remaining filediffs on diff 2 that don't match diff 1 are

shown as new changes in the interdiff.
There are some other fixes around this as well. Sorting now takes into

account the destination filename (which is what we display as the

primary filename for the diff), prioritizing the interdiff filename.

This fixes sorting during renames, and for changes introduced in a newer

diff revision. Note that this does impact sort order of diffs, which might

impact the flow of text written on any in-progress reviews that are on diffs

containing several moved files going into different locations (which is

pretty unlikely, and a fixable issue), but it's definitely more correct.
It also fixes the display of "File Reverted" for files. We weren't

computing this correctly when generating the master list of files for an

interdiff, affecting the file index.
Unit tests were added for all these conditions, including those in a

pathological diff affecting one of our customers.

All unit tests pass.
Tested with some standard interdiffs I test against, comparing against

the renders without this change. They matched up.
Tested against a couple pathological diffs that outright broke the diff

viewer before. They now render these nearly nonsensical diffs properly.