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/version.go

@@ -15,7 +15,7 @@ type GitVersion struct {
 }
 
 func GetGitVersion(osCommand *oscommands.OSCommand) (*GitVersion, error) {
-	versionStr, _, err := osCommand.Cmd.New(NewGitCmd("--version").ToString()).RunWithOutputs()
+	versionStr, _, err := osCommand.Cmd.New(NewGitCmd("--version").ToArgv()).RunWithOutputs()
 	if err != nil {
 		return nil, err
 	}