format-patch: assume --cover-letter for diff in multi-patch series
When we deal with a multi-patch series in git-format-patch(1), if we see `--interdiff` or `--range-diff` but no `--cover-letter`, we return with an error, saying: fatal: --range-diff requires --cover-letter or single patch or: fatal: --interdiff requires --cover-letter or single patch This makes sense because the cover-letter is where we place the diff from the previous version. However, considering that `format-patch` generates a multi-patch as needed, let's adopt a similar "cover as necessary" approach when using `--interdiff` or `--range-diff`. Therefore, relax the requirement for an explicit `--cover-letter` in a multi-patch series when the user says `--iterdiff` or `--range-diff`. Still, if only to return the error, respect "format.coverLetter=no" and `--no-cover-letter`. Signed-off-by: Rubén Justo <rjusto@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>