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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rjusto

builtin/log.c

@@ -2255,6 +2255,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	if (cover_letter == -1) {
 		if (config_cover_letter == COVER_AUTO)
 			cover_letter = (total > 1);
+		else if ((idiff_prev.nr || rdiff_prev) && (total > 1))
+			cover_letter = (config_cover_letter != COVER_OFF);
 		else
 			cover_letter = (config_cover_letter == COVER_ON);
 	}

t/t3206-range-diff.sh

@@ -545,6 +545,20 @@ do
 	'
 done
 
+test_expect_success "--range-diff implies --cover-letter for multi-patch series" '
+	test_when_finished "rm -f v2-000?-*" &&
+	git format-patch -v2 --range-diff=topic main..unmodified &&
+	test_grep "^Range-diff against v1:$" v2-0000-cover-letter.patch
+'
+
+test_expect_success "explicit --no-cover-letter defeats implied --cover-letter" '
+	test_when_finished "rm -f v2-000?-*" &&
+	test_must_fail git format-patch --no-cover-letter \
+		-v2 --range-diff=topic main..unmodified &&
+	test_must_fail git -c format.coverLetter=no format-patch \
+		-v2 --range-diff=topic main..unmodified
+'
+
 test_expect_success 'format-patch --range-diff as commentary' '
 	git format-patch --range-diff=HEAD~1 HEAD~1 >actual &&
 	test_when_finished "rm 0001-*" &&

t/t4014-format-patch.sh

@@ -2445,6 +2445,21 @@ test_expect_success 'interdiff: solo-patch' '
 	test_cmp expect actual
 '
 
+test_expect_success 'interdiff: multi-patch, implicit --cover-letter' '
+	test_when_finished "rm -f v23-0*.patch" &&
+	git format-patch --interdiff=boop~2 -2 -v23 &&
+	test_grep "^Interdiff against v22:$" v23-0000-cover-letter.patch &&
+	test_cmp expect actual
+'
+
+test_expect_success 'interdiff: explicit --no-cover-letter defeats implied --cover-letter' '
+	test_when_finished "rm -f v23-0*.patch" &&
+	test_must_fail git format-patch --no-cover-letter \
+		--interdiff=boop~2 -2 -v23 &&
+	test_must_fail git -c format.coverLetter=no format-patch \
+		--interdiff=boop~2 -2 -v23
+'
+
 test_expect_success 'format-patch does not respect diff.noprefix' '
 	git -c diff.noprefix format-patch -1 --stdout >actual &&
 	grep "^--- a/blorp" actual