Allow forks to use the same upgrade script, adds upstream repo if needed

tools/upgrade.sh

@@ -1,6 +1,19 @@
 current_path=`pwd`
 echo "Upgrading Oh My Zsh"
-( cd $ZSH && git pull origin master )
+
+git_branches=`git remote -v`
+default_origin=`echo $git_branches | egrep 'origin\s+https://github.com/robbyrussell/oh-my-zsh\.git'`
+upgrade_cmd="git pull origin master"
+if [ "N$default_origin" = "N" ]; then
+	upstream_branch=`echo $git_branches | egrep 'upstream\s+https://github.com/robbyrussell/oh-my-zsh\.git'`
+	if [ "N$upstream_branch" = "N" ]; then
+		upgrade_cmd="git remote add upstream https://github.com/robbyrussell/oh-my-zsh/; git fetch upstream; git merge upstream/master"
+	else
+		upgrade_cmd="git fetch upstream; git merge upstream/master"
+	fi
+fi
+
+( cd $ZSH && eval "$upgrade_cmd" )
 echo '         __                                     __  '
 echo '  ____  / /_     ____ ___  __  __   ____  _____/ /_ '
 echo ' / __ \/ __ \   / __ `__ \/ / / /  /_  / / ___/ __ \ '
@@ -9,4 +22,4 @@ echo '\____/_/ /_/  /_/ /_/ /_/\__, /    /___/____/_/ /_/  '
 echo '                        /____/'
 echo "Hooray! Oh My Zsh has been updated and/or is at the current version. \nAny new updates will be reflected when you start your next terminal session."
 echo "To keep up on the latest, be sure to follow Oh My Zsh on twitter: http://twitter.com/ohmyzsh"
-cd $current_path
+cd $current_path