[BZ#699023] indicate when local commits are unpushed

Submitted by Adam Dingle

Description

In git (and probably other distributed version control systems too) it's all too easy to make commits locally and then forget to push them to the server. When there are unpushed local commits, it would be nice if Meld would display an indication to that effect in the 'Status' column for the root directory of the local working copy. The text could read e.g. '2 local commits'.