t7501: add tests for --amend --signoff

Add tests for amending the commit to add Signed-off-by trailer. And
also to check if it does not add another trailer if one already exists.

Currently, there are tests for --signoff separately in t7501, however,
they are not tested with --amend.

Therefore, these tests belong with other similar tests of --amend in
t7501-commit-basic-functionality.

Helped-by: Phillip Wood <[email protected]>
Signed-off-by: Ghanshyam Thakkar <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: spectre10

t/t7501-commit-basic-functionality.sh

@@ -3,8 +3,7 @@
 # Copyright (c) 2007 Kristian Høgsberg <[email protected]>
 #
 
-# FIXME: Test the various index usages, test reflog,
-# signoff
+# FIXME: Test the various index usages, test reflog
 
 test_description='git commit'
 GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
@@ -462,6 +461,28 @@ test_expect_success 'amend commit to fix date' '
 
 '
 
+test_expect_success 'amend commit to add signoff' '
+
+	test_commit "msg" file content &&
+	git commit --amend --signoff &&
+	test_commit_message HEAD <<-EOF
+	msg
+
+	Signed-off-by: $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>
+	EOF
+'
+
+test_expect_success 'amend does not add signoff if it already exists' '
+
+	test_commit --signoff "tenor" file newcontent &&
+	git commit --amend --signoff &&
+	test_commit_message HEAD <<-EOF
+	tenor
+
+	Signed-off-by: $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>
+	EOF
+'
+
 test_expect_success 'commit mentions forced date in output' '
 	git commit --amend --date=2010-01-02T03:04:05 >output &&
 	grep "Date: *Sat Jan 2 03:04:05 2010" output