builtin/add: add ODB transaction around add_files_to_cache

The add_files_to_cache function is invoked internally by
builtin/commit.c and builtin/checkout.c for their flags that stage
modified files before doing the larger operation. These commands
can benefit from batched fsyncing.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/add.c

@@ -141,7 +141,16 @@ int add_files_to_cache(const char *prefix,
 	rev.diffopt.format_callback_data = &data;
 	rev.diffopt.flags.override_submodule_config = 1;
 	rev.max_count = 0; /* do not compare unmerged paths with stage #2 */
+
+	/*
+	 * Use an ODB transaction to optimize adding multiple objects.
+	 * This function is invoked from commands other than 'add', which
+	 * may not have their own transaction active.
+	 */
+	begin_odb_transaction();
 	run_diff_files(&rev, DIFF_RACY_IS_MODIFIED);
+	end_odb_transaction();
+
 	clear_pathspec(&rev.prune_data);
 	return !!data.add_errors;
 }