Added new 'push' command and 2-parameter form of 'add'.

Added new 'push' command and 2-parameter form of 'add'.

Now you can do:

	git subtree add --prefix=whatever git://wherever branchname

to add a new branch, instead of rather weirdly having to do 'git fetch'
first.  You can also split and push in one step:

	git subtree push --prefix=whatever git://wherever newbranch

(Somewhat cleaned up by apenwarr.)