alias: restore support for simple dotted aliases

alias: restore support for simple dotted aliases

Historically, config entries like alias.foo.bar expanded the alias
"foo.bar". The subsection-based alias syntax introduced in
ac1f12a9de (alias: support non-alphanumeric names via subsection
syntax, 2026-02-18) broke that behavior by treating such entries as
if they were subsection syntax.

Restore support for the old dotted form by falling back to the full
name when the final key is not "command". Add tests covering execution
and help output for simple dotted aliases.

Reported-by: Michael Grossfeld <michael.grossfeld@amd.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonatan Holmgren <jonatan@jontes.page>
Signed-off-by: Junio C Hamano <gitster@pobox.com>