Comment 4 for bug 391471

INADA Naoki, this bug partly affects bzr explorer.

I just tested, launching Diff these ways:
1) Diff icon beside the modified file
2) Diff icon in toolbar
3) Explore -> Browse Items -> Diff

None of these produced the error message. However Diff behavior WAS inconsistent:
(1) Launches built in diff instead of user default.
(2) Launches built in diff instead of user default.
(3) Launches the user configured diff.

I notice that when multiple files have changed, it may be preferable to launch the built in Diff OR use a different user defined command because:
a) Diff is launched once per modified file, in sequence (close one window, the next file opens). This may not be desirable behavior.
b) Some Diff apps have a different command syntax allowing multiple files to be opened at the same time. It would be nice to support this case.
c) When viewing changes to multiple files, rather than a single, specific file, the user is more likely wanting to view changes instead of edit them.

So, in conclusion, Diff should produce consistent behavior, launching the user specified default. Diff on multiple files should be handled separately to Diff of a single file for a better user experience.