help: use list_aliases() for alias listing

help: use list_aliases() for alias listing

help.c has its own get_alias() config callback that duplicates the
parsing logic in alias.c. Consolidate by teaching list_aliases() to
also store the alias values (via the string_list util field), then
use it in list_all_cmds_help_aliases() instead of the private
callback.

This preserves the existing error checking for value-less alias
definitions by checking in alias.c rather than help.c.

No functional change intended.

Signed-off-by: Jonatan Holmgren <jonatan@jontes.page>
Signed-off-by: Junio C Hamano <gitster@pobox.com>