After starting to edit a working tree file but later when your edit ends up identical to the original (this can also happen when you ran a wholesale regexp replace with something like "perl -i" that does not actually modify many of the paths), "git diff" between the index and the working tree outputs many "empty" diffs that show "diff --git" headers and nothing else, because these paths are stat-dirty. While it was a way to warn the user that the earlier action of the user made the index ineffective as an optimization mechanism, it was felt too loud for the purpose of warning even to experienced users, and also resulted in confusing people new to git.
This replaces the "empty" diffs with a single warning message at the end. Having many such paths hurts performance, and you can run "git-update-index --refresh" to update the lstat(2) information recorded in the index in such a case. "git-status" does so as a side effect, and that is more familiar to the end-user, so we recommend it to them.
The change affects only "git diff" that outputs patch text, because that is where the annoyance of too many "empty" diff is most strongly felt, and because the warning message can be safely ignored by downstream tools without getting mistaken as part of the patch. For the low-level "git diff-files" and "git diff-index", the traditional behaviour is retained.