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/commands/oscommands/os_test.go

@@ -10,13 +10,13 @@ import (
 
 func TestOSCommandRun(t *testing.T) {
 	type scenario struct {
-		command string
-		test    func(error)
+		args []string
+		test func(error)
 	}
 
 	scenarios := []scenario{
 		{
-			"rmdir unexisting-folder",
+			[]string{"rmdir", "unexisting-folder"},
 			func(err error) {
 				assert.Regexp(t, "rmdir.*unexisting-folder.*", err.Error())
 			},
@@ -25,7 +25,7 @@ func TestOSCommandRun(t *testing.T) {
 
 	for _, s := range scenarios {
 		c := NewDummyOSCommand()
-		s.test(c.Cmd.New(s.command).Run())
+		s.test(c.Cmd.New(s.args).Run())
 	}
 }