git-am: add am.threeWay config variable

Add the am.threeWay configuration variable to use the -3 or --3way
option of git am by default. When am.threeway is set and not desired
for a specific git am command, the --no-3way option can be used to
override it.

Signed-off-by: Remi Lespinet <remi.lespinet@ensimag.grenoble-inp.fr>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -152,6 +152,8 @@ static void am_state_init(struct am_state *state, const char *dir)
 
 	state->prec = 4;
 
+	git_config_get_bool("am.threeway", &state->threeway);
+
 	state->utf8 = 1;
 
 	git_config_get_bool("am.messageid", &state->message_id);