git: show alias info only with lone -h

git: show alias info only with lone -h

Builtin commands show usage information on stdout if called with -h as
their only option, usage.c::show_usage_if_asked() makes sure of that.

Aliases show alias information on stderr if called with -h as the first
option since a9a60b94cc (git.c: handle_alias: prepend alias info when
first argument is -h, 2018-10-09).  This is surprising when using
aliases for commands that take -h as a normal argument among others,
like git grep.

Tighten the condition and show the alias information only if -h is the
only option given, to be consistent with builtins.

It's probably still is a good idea to write to stderr, as an alias
command doesn't have to be a builtin and could instead produce output
with just -h that might be spoiled by an extra alias info line.

Reported-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>