Use insert overwrite for the first batch of seed (#149)

resolves #114

### Description

Uses `insert overwrite` for the first batch of seed to help seeds in external tables.

This must be a temporary fix and should follow dbt-labs/dbt-spark#182.

dbt/include/databricks/macros/materializations/seed.sql

@@ -2,6 +2,44 @@
   {{ return('%s') }}
 {% endmacro %}
 
+{% macro databricks__load_csv_rows(model, agate_table) %}
+
+  {% set batch_size = get_batch_size() %}
+  {% set column_override = model['config'].get('column_types', {}) %}
+
+  {% set statements = [] %}
+
+  {% for chunk in agate_table.rows | batch(batch_size) %}
+      {% set bindings = [] %}
+
+      {% for row in chunk %}
+          {% do bindings.extend(row) %}
+      {% endfor %}
+
+      {% set sql %}
+          insert {% if loop.index0 == 0 -%} overwrite {% else -%} into {% endif -%} {{ this.render() }} values
+          {% for row in chunk -%}
+              ({%- for col_name in agate_table.column_names -%}
+                  {%- set inferred_type = adapter.convert_type(agate_table, loop.index0) -%}
+                  {%- set type = column_override.get(col_name, inferred_type) -%}
+                    cast({{ get_binding_char() }} as {{type}})
+                  {%- if not loop.last%},{%- endif %}
+              {%- endfor -%})
+              {%- if not loop.last%},{%- endif %}
+          {%- endfor %}
+      {% endset %}
+
+      {% do adapter.add_query(sql, bindings=bindings, abridge_sql_log=True) %}
+
+      {% if loop.index0 == 0 %}
+          {% do statements.append(sql) %}
+      {% endif %}
+  {% endfor %}
+
+  {# Return SQL so we can render it out into the compiled files #}
+  {{ return(statements[0]) }}
+{% endmacro %}
+
 {% macro databricks__create_csv_table(model, agate_table) %}
   {%- set column_override = model['config'].get('column_types', {}) -%}
   {%- set quote_seed_column = model['config'].get('quote_columns', None) -%}