fix preview script so it can checkout and build from tags

s/cron/previews.sh

@@ -1,11 +1,18 @@
 #!/bin/sh
+
+function abort() {
+echo $1
+[ -z $2 ] && exit -1
+exit $2 # otherwise
+}
+
 [ -z ${ERESSEA} ] && ERESSEA=~/eressea
 branch="master"
 if [ -e ${ERESSEA}/build/.preview ]; then
     branch=`cat ${ERESSEA}/build/.preview`
 fi
 SRC=${ERESSEA}/git
-${SRC}/s/preview build ${branch}
+${SRC}/s/preview build ${branch} || abort "failed to build ${branch}"
 ${SRC}/s/preview version
 for game in 2 3 4 ; do
 	${SRC}/s/preview -g ${game} run && \

s/preview

@@ -22,8 +22,9 @@ exit $2 # otherwise
 function build() {
 assert_dir $SOURCE
 cd $SOURCE
+git fetch || abort "failed to update source. do you have local changes?"
 [ -z $1 ] || git checkout $1
-( git pull && git submodule update ) || abort "failed to update source. do you have local changes?"
+git submodule update
 s/build || abort "build failed."
 }