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/flow.go

@@ -49,13 +49,13 @@ func (self *FlowCommands) FinishCmdObj(branchName string) (oscommands.ICmdObj, e
 		return nil, errors.New(self.Tr.NotAGitFlowBranch)
 	}
 
-	cmdStr := NewGitCmd("flow").Arg(branchType, "finish", suffix).ToString()
+	cmdArgs := NewGitCmd("flow").Arg(branchType, "finish", suffix).ToArgv()
 
-	return self.cmd.New(cmdStr), nil
+	return self.cmd.New(cmdArgs), nil
 }
 
 func (self *FlowCommands) StartCmdObj(branchType string, name string) oscommands.ICmdObj {
-	cmdStr := NewGitCmd("flow").Arg(branchType, "start", name).ToString()
+	cmdArgs := NewGitCmd("flow").Arg(branchType, "start", name).ToArgv()
 
-	return self.cmd.New(cmdStr)
+	return self.cmd.New(cmdArgs)
 }