provisioning: Don't rebuild DB if running migrations is sufficient.

This results in a significant optimization in the performance of
re-provisioning Zulip if all that you're doing is rebasing onto a
newer version of master (which just adds new migrations).

The change carries some risk of generating unpleasant-to-debug
situations, because if we merge a buggy migration and then later fix
it, some clients may not have a properly migrated database (and also,
this changes how populate_db commutes with migrations).  But it seems
worth it, given how much time is currently wasted by not having this.

Fixes: #9512.

tools/lib/provision.py

@@ -321,7 +321,7 @@ def main(options):
         import django
         django.setup()
 
-        from zerver.lib.test_fixtures import template_database_status
+        from zerver.lib.test_fixtures import template_database_status, run_db_migrations
 
         try:
             from zerver.lib.queue import SimpleQueueClient
@@ -344,6 +344,8 @@ def main(options):
         if options.is_force or dev_template_db_status == 'needs_rebuild':
             run(["tools/setup/postgres-init-dev-db"])
             run(["tools/do-destroy-rebuild-database"])
+        elif dev_template_db_status == 'run_migrations':
+            run_db_migrations('dev')
         elif dev_template_db_status == 'current':
             print("No need to regenerate the dev DB.")
 
@@ -351,6 +353,8 @@ def main(options):
         if options.is_force or test_template_db_status == 'needs_rebuild':
             run(["tools/setup/postgres-init-test-db"])
             run(["tools/do-destroy-rebuild-test-database"])
+        elif test_template_db_status == 'run_migrations':
+            run_db_migrations('test')
         elif test_template_db_status == 'current':
             print("No need to regenerate the test DB.")