builtin/help.c: split "-a" processing into two

builtin/help.c: split "-a" processing into two

"help -a" (help all) gives the list of available commands and then
further gives hints on the use of "git help".   Separate these into
two steps, because we will add "help -g" (help guides) that want to
also show the overall hints after it is done.

While at it, change the definition of the "-a" option to use OPT_BOOL,
not the deprecated OPT_BOOLEAN.  We do not behave differently when
the user gives the "-a" option multiple times, e.g. "git help -a -a".

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>