diff-update-on-the-flydiff-update-on-the-fly is a variable defined in `
diff-mode.el'. Its value is
tDocumentation: Non-nil means hunk headers are kept up-to-date on-the-fly. When editing a diff file, the line numbers in the hunk headers need to be kept consistent with the actual diff. This can either be done on the fly (but this sometimes interacts poorly with the undo mechanism) or whenever the file is written (can be slow when editing big diffs). You can customize this variable.