[PATCH v3] format-patch: assume --cover-letter for diff in multi-patch series

["Rubén Justo" <rjusto@gmail.com>]

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>
---

Range-diff against v2:
1:  ff67f24022 ! 1:  8dc5f16d83 format-patch: assume --cover-letter for diff in multi-patch series
    @@ t/t3206-range-diff.sh: do
      done
      
     +test_expect_success "format-patch --range-diff, implicit --cover-letter" '
    ++	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 &&
     +	git format-patch -v2 --range-diff=topic main..unmodified &&
     +	test_when_finished "rm v2-000?-*" &&
    -+	test_grep "^Range-diff against v1:$" v2-0000-*
    ++	test_grep "^Range-diff against v1:$" v2-0000-cover-letter.patch
     +'
     +
      test_expect_success 'format-patch --range-diff as commentary' '
    @@ t/t4014-format-patch.sh: test_expect_success 'interdiff: solo-patch' '
      '
      
     +test_expect_success 'interdiff: multi-patch, implicit --cover-letter' '
    ++	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 &&
     +	git format-patch --interdiff=boop~2 -2 -v23 &&
     +	test_grep "^Interdiff against v22:$" v23-0000-cover-letter.patch &&
     +	test_cmp expect actual

 builtin/log.c           |  2 ++
 t/t3206-range-diff.sh   | 10 ++++++++++
 t/t4014-format-patch.sh | 10 ++++++++++
 3 files changed, 22 insertions(+)

diff --git a/builtin/log.c b/builtin/log.c
index c8ce0c0d88..8032909d4f 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -2277,6 +2277,8 @@ 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);
 	}
diff --git a/t/t3206-range-diff.sh b/t/t3206-range-diff.sh
index 7b05bf3961..4a597466a2 100755
--- a/t/t3206-range-diff.sh
+++ b/t/t3206-range-diff.sh
@@ -545,6 +545,16 @@ do
 	'
 done
 
+test_expect_success "format-patch --range-diff, implicit --cover-letter" '
+	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 &&
+	git format-patch -v2 --range-diff=topic main..unmodified &&
+	test_when_finished "rm v2-000?-*" &&
+	test_grep "^Range-diff against v1:$" v2-0000-cover-letter.patch
+'
+
 test_expect_success 'format-patch --range-diff as commentary' '
 	git format-patch --range-diff=HEAD~1 HEAD~1 >actual &&
 	test_when_finished "rm 0001-*" &&
diff --git a/t/t4014-format-patch.sh b/t/t4014-format-patch.sh
index ba85b582c5..b96348eebd 100755
--- a/t/t4014-format-patch.sh
+++ b/t/t4014-format-patch.sh
@@ -2492,6 +2492,16 @@ test_expect_success 'interdiff: solo-patch' '
 	test_cmp expect actual
 '
 
+test_expect_success 'interdiff: multi-patch, implicit --cover-letter' '
+	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 &&
+	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 'format-patch does not respect diff.noprefix' '
 	git -c diff.noprefix format-patch -1 --stdout >actual &&
 	grep "^--- a/blorp" actual
-- 
2.45.2.410.g52d620e86a