Maintenance tasks other than "gc" now properly go background when
"git maintenance" runs them.
* ps/maintenance-detach-fix:
run-command: fix detaching when running auto maintenance
builtin/maintenance: add a `--detach` flag
builtin/gc: add a `--detach` flag
builtin/gc: stop processing log file on signal
builtin/gc: fix leaking config values
builtin/gc: refactor to read config into structure
config: fix constness of out parameter for `git_config_get_expiry()`