t7501: add tests for --amend --signoff

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 <redacted>
Signed-off-by: Ghanshyam Thakkar <redacted>
Signed-off-by: Junio C Hamano <redacted>