help: use early config when autocorrecting aliases

Git has this feature which suggests similar commands (including aliases)
in case the user specified an unknown command.

This feature currently relies on a side effect of the way we expand
aliases right now: when a command is not a builtin, we use the regular
config machinery (meaning: discovering the .git/ directory and
initializing global state such as the config cache) to see whether the
command refers to an alias.

However, we will change the way aliases are expanded in the next
commits, to use the early config instead. That means that the
autocorrect feature can no longer discover the available aliases by
looking at the config cache (because it has not yet been initialized).

So let's just use the early config machinery instead.

This is slightly less performant than the previous way, as the early
config is used *twice*: once to see whether the command refers to an
alias, and then to see what aliases are most similar. However, this is
hardly a performance-critical code path, so performance is less important
here.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -330,7 +330,7 @@ const char *help_unknown_cmd(const char *cmd)
 	memset(&other_cmds, 0, sizeof(other_cmds));
 	memset(&aliases, 0, sizeof(aliases));
 
-	git_config(git_unknown_cmd_config, NULL);
+	read_early_config(git_unknown_cmd_config, NULL);
 
 	load_command_list("git-", &main_cmds, &other_cmds);