seed: Add new macro get_csv_sql

The implementation of this new macro `get_csv_sql` comes as a suggestion
by @jtcohen6 at
dbt-labs/dbt-snowflake#112 (comment).

The underlying reason is that the `dbt-snowflake` package needs to run
these queries inside a transaction, but with the current implementation,
the fix would require to duplicate the entire default seed
materialization logic in the package.

core/dbt/include/global_project/macros/materializations/seeds/helpers.sql

@@ -44,6 +44,17 @@
 {% endmacro %}
 
 
+{% macro get_csv_sql(create_or_truncate_sql, insert_sql) %}
+ {{ adapter.dispatch('get_csv_sql', 'dbt')(create_or_truncate_sql, insert_sql) }}
+{% endmacro %}
+
+{% macro default__get_csv_sql(create_or_truncate_sql, insert_sql) %}
+ {{ create_or_truncate_sql }};
+ -- dbt seed --
+ {{ insert_sql }}
+{% endmacro %}
+
+
 {% macro get_binding_char() -%}
  {{ adapter.dispatch('get_binding_char', 'dbt')() }}
 {%- endmacro %}

core/dbt/include/global_project/macros/materializations/seeds/seed.sql

@@ -31,9 +31,7 @@
  {% set sql = load_csv_rows(model, agate_table) %}
 
  {% call noop_statement('main', code ~ ' ' ~ rows_affected, code, rows_affected) %}
- {{ create_table_sql }};
- -- dbt seed --
- {{ sql }}
+ {{ get_csv_sql(create_table_sql, sql) }};
  {% endcall %}
 
  {% set target_relation = this.incorporate(type='table') %}