maintenance: replace run_auto_gc()

The run_auto_gc() method is used in several places to trigger a check
for repo maintenance after some Git commands, such as 'git commit' or
'git fetch'.

To allow for extra customization of this maintenance activity, replace
the 'git gc --auto [--quiet]' call with one to 'git maintenance run
--auto [--quiet]'. As we extend the maintenance builtin with other
steps, users will be able to select different maintenance activities.

Rename run_auto_gc() to run_auto_maintenance() to be clearer what is
happening on this call, and to expose all callers in the current diff.
Rewrite the method to use a struct child_process to simplify the calls
slightly.

Since 'git fetch' already allows disabling the 'git gc --auto'
subprocess, add an equivalent option with a different name to be more
descriptive of the new behavior: '--[no-]maintenance'. Update the
documentation to include these options at the same time.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 69ac78d..f9b0a0c 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1702,7 +1702,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	git_test_write_commit_graph_or_die();
 
 	repo_rerere(the_repository, 0);
-	run_auto_gc(quiet);
+	run_auto_maintenance(quiet);
 	run_commit_hook(use_editor, get_index_file(), "post-commit", NULL);
 	if (amend && !no_post_rewrite) {
 		commit_post_rewrite(the_repository, current_head, &oid);