Add ability for graph squash context menu to work on non-contiguous commits

Add ability for Squash context menu in log view to work for non-consecutive commits. Currently it only works for consecutive commits. If non-consecutive commits are chosen then smartgit would trigger a interactive rebase that does the following:

- reorder selected commits so they are consecutive

- then squash them

If there is an merge issue then smartgit drops into interactive rebase resolve step so use can resolve (or cancel).