Proposal: Add tip to use git update-git-for-windows when running git help or git version on Windows

I've recently started exploring git for windows, and one thing I noticed was, there was no sign of updates! I googled a bit and found the git update-git-for-windows command. I think it would be nice if we could add a line like:

"Tip: Run `git update-git-for-windows` to update git"

At the last of the output when we run git help, git version or both.

Looking at the code, adding a compiler flag to check for windows in help.c and print it seems to be a possible solution.