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/git_commands/file_loader.go

@@ -82,14 +82,14 @@ type FileStatus struct {
 }
 
 func (c *FileLoader) gitStatus(opts GitStatusOptions) ([]FileStatus, error) {
-	cmdStr := NewGitCmd("status").
+	cmdArgs := NewGitCmd("status").
 		Arg(opts.UntrackedFilesArg).
 		Arg("--porcelain").
 		Arg("-z").
 		ArgIf(opts.NoRenames, "--no-renames").
-		ToString()
+		ToArgv()
 
-	statusLines, _, err := c.cmd.New(cmdStr).DontLog().RunWithOutputs()
+	statusLines, _, err := c.cmd.New(cmdArgs).DontLog().RunWithOutputs()
 	if err != nil {
 		return []FileStatus{}, err
 	}