[almost-patch] Add diff regular expressions for Markdown and Zshell

Hi. Git comes with a few of the diff-regexes built-in. They are listed here: https://git-scm.com/docs/gitattributes#_defining_a_custom_hunk_header

I'm a Zsh plugin creator and I've found the following regexes to be useful:

- Zshell function-matching:
    xfuncname =
"^((function[[:blank:]]+[^[:blank:]]+[[:blank:]]*(\\(\\)|))|([^[:blank:]]+[[:blank:]]*\\(\\)))[[:blank:]]*(\\{|)[[:blank:]]*$"

- Markdown hading-matching:
    xfuncname = "^#+[[:blank:]].*$"

Could those regexes be added to the built-in set? I think that they're extremely useful and shipping them with Git would lift the requirement of having them in each .gitconfig on each machine – particularly useful for e.g.: receiving bug-reports from Zsh-plugin users, that may have the patterns not set up.