t4150: tests for am --[no-]scissors

t4150: tests for am --[no-]scissors

Since 017678b (am/mailinfo: Disable scissors processing by default,
2009-08-26), git-am supported the --[no-]scissors option, passing it to
git-mailinfo.

Add tests to ensure that git-am will pass the --scissors option to
git-mailinfo, and that --no-scissors will override the configuration
setting of mailinfo.scissors.

Reviewed-by: Stefan Beller <redacted>
Reviewed-by: Johannes Schindelin <redacted>
Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>