merge: add a --signoff flag

Some projects require every commit, even merges, to be signed off
[*1*].  Because "git merge" does not have a "--signoff" option like
"git commit" does, the user needs to add one manually when the
command presents an editor to describe the merge, or later use "git
commit --amend --signoff".

Help developers of these projects by teaching "--signoff" option to
"git merge".

*1* https://public-inbox.org/git/CAHv71zK5SqbwrBFX=a8-DY9H3KT4FEyMgv__p2gZzNr0WUAPUw@mail.gmail.com/T/#u

Requested-by: Dan Kohn <dan@linuxfoundation.org>
Signed-off-by: Łukasz Gryglicki <lukaszgryglicki@o2.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -64,6 +64,14 @@ OPTIONS
 -------
 include::merge-options.txt[]
 
+--signoff::
+	Add Signed-off-by line by the committer at the end of the commit
+	log message.  The meaning of a signoff depends on the project,
+	but it typically certifies that committer has
+	the rights to submit this work under the same license and
+	agrees to a Developer Certificate of Origin
+	(see http://developercertificate.org/ for more information).
+
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
 	GPG-sign the resulting merge commit. The `keyid` argument is

builtin/merge.c

@@ -70,6 +70,7 @@ static int continue_current_merge;
 static int allow_unrelated_histories;
 static int show_progress = -1;
 static int default_to_upstream = 1;
+static int signoff;
 static const char *sign_commit;
 
 static struct strategy all_strategy[] = {
@@ -233,6 +234,7 @@ static struct option builtin_merge_options[] = {
 	{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, N_("key-id"),
 	  N_("GPG sign commit"), PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
 	OPT_BOOL(0, "overwrite-ignore", &overwrite_ignore, N_("update ignored files (default)")),
+	OPT_BOOL(0, "signoff", &signoff, N_("add Signed-off-by:")),
 	OPT_END()
 };
 
@@ -763,6 +765,8 @@ static void prepare_to_commit(struct commit_list *remoteheads)
 	strbuf_addch(&msg, '\n');
 	if (0 < option_edit)
 		strbuf_commented_addf(&msg, _(merge_editor_comment), comment_line_char);
+	if (signoff)
+		append_signoff(&msg, ignore_non_trailer(msg.buf, msg.len), 0);
 	write_file_buf(git_path_merge_msg(), msg.buf, msg.len);
 	if (run_commit_hook(0 < option_edit, get_index_file(), "prepare-commit-msg",
 			    git_path_merge_msg(), "merge", NULL))

t/t7614-merge-signoff.sh

@@ -0,0 +1,69 @@
+#!/bin/sh
+
+test_description='git merge --signoff
+
+This test runs git merge --signoff and makes sure that it works.
+'
+
+. ./test-lib.sh
+
+# Setup test files
+test_setup() {
+	# Expected commit message after merge --signoff
+	cat >expected-signed <<EOF &&
+Merge branch 'master' into other-branch
+
+Signed-off-by: $(git var GIT_COMMITTER_IDENT | sed -e "s/>.*/>/")
+EOF
+
+	# Expected commit message after merge without --signoff (or with --no-signoff)
+	cat >expected-unsigned <<EOF &&
+Merge branch 'master' into other-branch
+EOF
+
+	# Initial commit and feature branch to merge master into it.
+	git commit --allow-empty -m "Initial empty commit" &&
+	git checkout -b other-branch &&
+	test_commit other-branch file1 1
+}
+
+# Setup repository, files & feature branch
+# This step must be run if You want to test 2,3 or 4
+# Order of 2,3,4 is not important, but 1 must be run before
+# For example `-r 1,4` or `-r 1,4,2 -v` etc
+# But not `-r 2` or `-r 4,3,2,1`
+test_expect_success 'setup' '
+	test_setup
+'
+
+# Test with --signoff flag
+test_expect_success 'git merge --signoff adds a sign-off line' '
+	git checkout master &&
+	test_commit master-branch-2 file2 2 &&
+	git checkout other-branch &&
+	git merge master --signoff --no-edit &&
+	git cat-file commit HEAD | sed -e "1,/^\$/d" >actual &&
+	test_cmp expected-signed actual
+'
+
+# Test without --signoff flag
+test_expect_success 'git merge does not add a sign-off line' '
+	git checkout master &&
+	test_commit master-branch-3 file3 3 &&
+	git checkout other-branch &&
+	git merge master --no-edit &&
+	git cat-file commit HEAD | sed -e "1,/^\$/d" >actual &&
+	test_cmp expected-unsigned actual
+'
+
+# Test for --no-signoff flag
+test_expect_success 'git merge --no-signoff flag cancels --signoff flag' '
+	git checkout master &&
+	test_commit master-branch-4 file4 4 &&
+	git checkout other-branch &&
+	git merge master --no-edit --signoff --no-signoff &&
+	git cat-file commit HEAD | sed -e "1,/^\$/d" >actual &&
+	test_cmp expected-unsigned actual
+'
+
+test_done