Fix running postgres-init-db via a relative path.

If the user runs ./scripts/setup/postgres-init-db, then dirname "$0"
would no longer refer to the correct directory after cd /.

scripts/setup/postgres-init-db

@@ -2,9 +2,6 @@
 set -e
 set -x
 
-# Make sure the current working directory is readable
-cd /
-
 # Shut down all services to ensure a quiescent state.
 supervisorctl stop all
 
@@ -12,6 +9,10 @@ supervisorctl stop all
 # source because postgres user can't read /root/zulip/scripts/setup.
 source "$(dirname "$0")/terminate-psql-sessions" postgres zulip zulip_base
 
+(
+# Make sure the current working directory is readable by postgres
+cd /
+
 su postgres -c psql <<EOF
 CREATE USER zulip;
 ALTER ROLE zulip SET search_path TO zulip,public;
@@ -23,6 +24,7 @@ su postgres -c 'psql zulip' <<EOF
 CREATE SCHEMA zulip AUTHORIZATION zulip;
 CREATE EXTENSION tsearch_extras SCHEMA zulip;
 EOF
+)
 
 # Clear memcached to avoid contamination from previous database state
 sh "$(dirname "$0")/flush-memcached"