Add rough documentation of using git with GCC

[Joseph Myers]

On Fri, 10 Jan 2020, Joseph Myers wrote:

> In gitwrite.html: the various custom setup of checkouts for which scripts 
> have been posted, including usage of branches not fetched by default; an 
> actual example commit session should be included like the one in 
> svnwrite.html; more examples should be given of git error messages, such 
> as if you try to push when someone else has pushed to the same branch or 
> if you try to pull with local changes to a file that's also been changed 
> upstream ("git stash" should probably be mentioned somewhere); setting up 
> git-merge-changelog should be discussed; there should be instructions for 
> rebasing a branch that was originally created in the git-svn mirror; 
> appropriate formatting of commit messages.

And we should also mention configuring your email address for git, if you 
haven't used git on that system before or the default address you've 
configured for git isn't the one you want for GCC commits.

-- 
Joseph S. Myers
joseph@codesourcery.com