pretty: Respect --abbrev option

Prior to this, the output of git log -1 --format=%h was always 7
characters long, without regard to whether --abbrev had been passed.

Signed-off-by: Will Palmer <wmpalmer@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

shortlog.h

@@ -12,6 +12,7 @@ struct shortlog {
 	int in1;
 	int in2;
 	int user_format;
+	int abbrev;
 
 	char *common_repo_prefix;
 	int email;