Construct arg vector manually rather than parse string

By constructing an arg vector manually, we no longer need to quote arguments

Mandate that args must be passed when building a command

Now you need to provide an args array when building a command.
There are a handful of places where we need to deal with a string,
such as with user-defined custom commands, and for those we now require
that at the callsite they use str.ToArgv to do that. I don't want
to provide a method out of the box for it because I want to discourage its
use.

For some reason we were invoking a command through a shell when amending a
commit, and I don't believe we needed to do that as there was nothing user-
supplied about the command. So I've switched to using a regular command out-
side the shell there

pkg/app/app.go

@@ -191,7 +191,7 @@ func (app *App) setupRepo() (bool, error) {
 		}
 
 		if shouldInitRepo {
-			if err := app.OSCommand.Cmd.New("git init " + initialBranchArg).Run(); err != nil {
+			if err := app.OSCommand.Cmd.New([]string{"git", "init", initialBranchArg}).Run(); err != nil {
 				return false, err
 			}
 			return false, nil