Merge branch 'fc/pull-merge-rebase'

When a user does not tell "git pull" to use rebase or merge, the command gives a loud message telling a user to choose between rebase or merge but creates a merge anyway, forcing users who would want to rebase to redo the operation. Fix an early part of this problem by tightening the condition to give the message---there is no reason to stop or force the user to choose between rebase or merge if the history fast-forwards. * fc/pull-merge-rebase: pull: display default warning only when non-ff pull: correct condition to trigger non-ff advice pull: get rid of unnecessary global variable pull: give the advice for choosing rebase/merge much later pull: refactor fast-forward check