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.go

@@ -45,7 +45,7 @@ func (self *FileCommands) GetEditCmdStrLegacy(filename string, lineNumber int) (
 		editor = self.os.Getenv("EDITOR")
 	}
 	if editor == "" {
-		if err := self.cmd.New("which vi").DontLog().Run(); err == nil {
+		if err := self.cmd.New([]string{"which", "vi"}).DontLog().Run(); err == nil {
 			editor = "vi"
 		}
 	}