Added pull method to gitconfig

Currently, when pulling I get the following error in git:

hint: Pulling without specifying how to reconcile divergent branches is
hint: discouraged. You can squelch this message by running one of the following
hint: commands sometime before your next pull:
hint: 
hint:   git config pull.rebase false  # merge (the default strategy)
hint:   git config pull.rebase true   # rebase
hint:   git config pull.ff only       # fast-forward only
hint: 
hint: You can replace "git config" with "git config --global" to set a default
hint: preference for all repositories. You can also pass --rebase, --no-rebase,
hint: or --ff-only on the command line to override the configured default per
hint: invocation.
Already up to date.

In order to fix this, I'm adding here the following lines to the standard gitconfig file:

[pull]
  rebase = true

I think this would be the desired behaviour, but I'm also open to discussing if we want to use any of the other two (merge, ff).

\approve @marius @ds