save modified file-visiting buffers more often

    Previously we did save, or offer to save such buffers when:
    
    * Before refreshing (g) the status buffer.  (Which also happens when
      refreshing some other Magit buffer, but unfortunately at the wrong
      time; after updating the other buffer.)
    * Before creating or jumping to the status buffer (C-x g).
    * Before creating and/or checking out a branch.
    
    Now we do so when:
    
    * Before refreshing any Magit buffer.
    * Before creating or jumping to any Magit buffer.
    * Before calling git in `magit-call-git' and `magit-start-git',
      which means that we do so for many more Magit commands, which
      end up running Git using one of these functions.
    
    This is controlled using the following new hooks:
    
    * magit-mode-setup-hook
    * magit-pre-refresh-hook
    * magit-pre-call-git-hook
    * magit-pre-start-git-hook

Dear `next`-users. Please set `magit-save-repository-buffers` to t (the default, but you might have changed it to `dontask`) and report here, when you run into situations when this kicks in but shouldn't, or does not but should.