I use the Diff bundle a lot. I regularly do a 'svn diff | mate' command prior to check-ins. One of the things I typically do with larger sets of files is collapse all the diffs so there is one file per line. I can then examine each group of changes, and once happy with the review (or as they get committed), I will delete the collapsed "line".
This is a tedious process because the foldings for a diff are not automatically done for me. This is one of many cases I've found where the folding separators cannot be described properly since the close of the fold is not a match for anything in particular, just the fact that a new fold is starting. (Another such case is for Markdown-- I'd like to provide foldings for the "# ..." header, down to the next available "# ..." header, but I can't express that.)
So what I do is start at the top of the file, then hit Option+Shift +⇣ which will select just that one file's diff somehow (I haven't investigated how TextMate knows to skip down to the next "Index..." line like that. Does it do so based on the lines identified as separators?). Then I hit ⌘+F1 to fold the selection.
Again this is very tedious, especially for a large diff. I would be very happy if someone found a way to automate this process. Ideally, it should be possible to describe a folding marker whose end is identified by the start of a new fold, but I don't think it's possible today.
-Brad