To answer the first part, you can do this by opening the folder in SB2:
- File -> Open Folder...
- Open your local git repository (The directory where the .git file is located)
- Commands like 'Git: Add all' should be available via cntrl+shift+p
To answer the second part, the code you entered is for when you want to use git bash as your git client. This code will set up SB2 as your text editor for git bash. If it worked, the next time you make a commit in git bash, just type 'git commit' and SublimeText2 should open up, to where you can enter your commit message.
If you need to revert back your core.editor to default, look at the following: Git: How can I reset a config parameter after I have accidentally changed it?
To try and answer your comment,
What's the difference between git bash and the git plugin for SB2?
They are just different ways to interact with git and git repositories.
Thus you would use the SB2 plugin as a replacement for git bash. This means you would be doing all the git commands (git add, git commit, etc) via the command palette in SB2, rather than as a command line via git bash(and thus no, you would not have to always start with git bash until you 'git commit'). If you wanted to use the git plugin for SB2, you wouldn't need to make SB2 your default editor for git bash(second part) as the plugin will always use SB2 for this purpose.
If you want to interact with git via git bash instead of the SB2 plugin, you would not need to install the git plugin on SB2. To give some background information, by default Git Bash usually uses a command line text editor(VI or VIM(https://en.wikipedia.org/wiki/Vim_(text_editor))). Git Bash will then use this text editor(VIM for example), when it needs user input(like when you enter 'git commit'). If you are not familiar with these text editors you can change the editor git bash uses(core.editor) to something like SB2((second part)the code you entered in git bash).
This is entirely my opinion, but I think it would be simpler just to start out by learning to use git via git bash as then you can more closely follow git tutorials like the following(which is a good place to start): http://git-scm.com/docs/gittutorial. Also, you then can still use SB2 to edit the files within your git repo, though aren't restricted to only using SB2 and can use any text editor or IDE in the future with git bash.