Create --force-with-lease equivalent checkbox on push dialog

[jm4R]

Currently we have force checkbox, but force-with-lease is safer alternative and in most cases should be used instead:
https://git-scm.com/docs/git-push#Documentation/git-push.txt---force-with-leaseltrefnamegt

I think the version without additional arguments will be enough for now.

[davvid]

Sounds good. Let's dig up the version number where the feature appeared and we can add a force-with-lease version value to the _version table in cola/version.py.

When self.action == 'PUSH' and version.check_git('force-with-lease') is true we can use 'force_with_lease', and 'force' otherwise.

https://github.com/git-cola/git-cola/blob/master/cola/widgets/remote.py#L519