bash completion: Add completion for 'git help'

bash completion: Add completion for 'git help'

Rename cached __git_commandlist to __git_porcelain_commandlist and
add __git_all_commandlist that only filters out *--* helpers.

Completions for 'git help' will use the __git_all_commandlist, while
the __git_porcelain_commandlist is used for git command completion.

Users who actually read man pages may want to see help for plumbing
commands.

Signed-off-by: Lee Marlow <lee.marlow@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>