Eskil

Make a plugin system for directory diff, similar as for file diff.
A plugin can do file compare and gets:
Two read streams.
Info about files (names, size)
Info about preferences
Returns "not equal", "equal", or "ok so far".
"ok so far" would tell normal diff to continue with the streams
as normal.

A plugin could be allowed to process file names for matching between
dirs. E.g. if a certain rename pattern has happened, it could help with
that.

The first part, doing file compare in a plugin works.

Keeping open for second suggestion.