removed a hardcoded 'origin' in git.zsh and added new function for number of commits ahead of remote

There was a hardcoded 'origin' to get the remote branch, but this won't work for all setups. Therefor added @{upstream} which will automatically get the upstream branch. Works since git 1.7.0.
Additionally added a function which returns the exact number of commits ahead of the upstream branch.
And added .DS_Store to .gitignore for proper OS X setup...