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/updates/updates.go

@@ -283,7 +283,7 @@ func (u *Updater) downloadAndInstall(rawUrl string) error {
 	}
 
 	u.Log.Info("untarring tarball/unzipping zip file")
-	err = u.OSCommand.Cmd.New(fmt.Sprintf("tar -zxf %s %s", u.OSCommand.Quote(zipPath), "lazygit")).Run()
+	err = u.OSCommand.Cmd.New([]string{"tar", "-zxf", zipPath, "lazygit"}).Run()
 	if err != nil {
 		return err
 	}