contrib/rerere-train: avoid useless gpg sign in training

Users may have configured "git merge" to always require GPG
signing the resulting commits. We are not running "git merge" to
re-create merge commits, but merely to replay merge conflicts,
and we will immediately discard the resulting commits; there
is no point in signing them.

Override such configuration that forces useless signing from the
command line with the "--no-gpg-sign" option.

Signed-off-by: Celeste Liu <coelacanthus@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/rerere-train.sh

@@ -75,7 +75,7 @@ do
 		continue
 	fi
 	git checkout -q "$parent1^0"
-	if git merge $other_parents >/dev/null 2>&1
+	if git merge --no-gpg-sign $other_parents >/dev/null 2>&1
 	then
 		# Cleanly merges
 		continue