Make message_markdown_help configurable

[jwilk]

I don't find the current message_markdown_help very helpful.
I'd prefer if it reminded me inline about these Markdown syntax aspects that I tend to forget. (Is it [text](URL) or [URL](text) or (text)[URL] or...? It's impossible for me to remember.)
But I don't think you could craft one message that would make everybody happy.
So could you make message_markdown_help configurable?

[leandro-lucarella-sociomantic]

Do you know if the git config file supports multi-line values?

[leandro-lucarella-sociomantic]

Apparently it can. It should be quite easy to implement as just a new git config variable, although I would keep it as a rather obscure feature, only document it in the man page and let the user use git config himself if he wants to override this message.