[git] Make .gitignore ignore everything except a few files

There are a bunch of similar questions about this, so I'll post what I wrote before:

The only way I got this to work on my machine was to do it this way:

# Ignore all directories, and all sub-directories, and it's contents:
*/*

#Now ignore all files in the current directory 
#(This fails to ignore files without a ".", for example 
#'file.txt' works, but 
#'file' doesn't):
*.*

#Only Include these specific directories and subdirectories and files if you wish:
!wordpress/somefile.jpg
!wordpress/
!wordpress/*/
!wordpress/*/wp-content/
!wordpress/*/wp-content/themes/
!wordpress/*/wp-content/themes/*
!wordpress/*/wp-content/themes/*/*
!wordpress/*/wp-content/themes/*/*/*
!wordpress/*/wp-content/themes/*/*/*/*
!wordpress/*/wp-content/themes/*/*/*/*/*

Notice how you have to explicitly allow content for each level you want to include. So if I have subdirectories 5 deep under themes, I still need to spell that out.

This is from @Yarin's comment here: https://stackoverflow.com/a/5250314/1696153

These were useful topics:

I also tried

*
*/*
**/**

and **/wp-content/themes/**

or /wp-content/themes/**/*

None of that worked for me, either. Lots of trail and error!