builtin-help: make some internal functions available to other builtins

builtin-help: make some internal functions available to other builtins

Make load_command_list() capable of filtering for a given prefix and
loading into a pair of "struct cmdnames" supplied by the caller.

Make the static add_cmdname(), exclude_cmds() and is_in_cmdlist()
functions non-static.

Make list_commands() accept a custom title, and work from a pair of
"struct cmdnames" supplied by the caller.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>