merge: add config option for verifySignatures

merge: add config option for verifySignatures

git merge --verify-signatures can be used to verify that the tip commit
of the branch being merged in is properly signed, but it's cumbersome to
have to specify that every time.

Add a configuration option that enables this behaviour by default, which
can be overridden by --no-verify-signatures.

Signed-off-by: Hans Jerry Illikainen <redacted>
Signed-off-by: Junio C Hamano <redacted>