Change "git reset" default to --mixed

Calling "git reset" on the command line (without further arguments) defaults to
--mixed, which is reason enough to make it the default for us, too.

But I also find myself using --mixed more often than --soft. The main use case
for me is that I made a bunch of WIP commits, and want to turn them into real
commits when I'm done hacking. I select the last commit before the WIP commits
and reset to it, leaving all changes of all those commits in the working
directory. Since I want to start staging things from there, I prefer those
modifications to be unstaged at that point, which is what --mixed does.

pkg/gui/controllers/helpers/refs_helper.go

@@ -169,8 +169,8 @@ func (self *RefsHelper) CreateGitResetMenu(ref string) error {
 	}
 	strengths := []strengthWithKey{
 		// not i18'ing because it's git terminology
-		{strength: "soft", label: "Soft reset", key: 's'},
 		{strength: "mixed", label: "Mixed reset", key: 'm'},
+		{strength: "soft", label: "Soft reset", key: 's'},
 		{strength: "hard", label: "Hard reset", key: 'h'},
 	}