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/remote_loader.go

@@ -31,8 +31,8 @@ func NewRemoteLoader(
 }
 
 func (self *RemoteLoader) GetRemotes() ([]*models.Remote, error) {
-	cmdStr := NewGitCmd("branch").Arg("-r").ToString()
-	remoteBranchesStr, err := self.cmd.New(cmdStr).DontLog().RunWithOutput()
+	cmdArgs := NewGitCmd("branch").Arg("-r").ToArgv()
+	remoteBranchesStr, err := self.cmd.New(cmdArgs).DontLog().RunWithOutput()
 	if err != nil {
 		return nil, err
 	}