git: read local config in --list-cmds

git: read local config in --list-cmds

Normally code that is checking config before we've decided to do
setup_git_directory() would use read_early_config(), which uses
discover_git_directory() to tentatively see if we're in a repo,
and if so to add it to the config sequence.

But list_cmds() uses the caching configset mechanism which
rightly does not use read_early_config(), because it has no
idea if it's being called early.

Call setup_git_directory_gently() so we can pick up repo-level
config (like completion.commands).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>