Retry on index.lock error

I don't know why we're getting index.lock errors but they're impossile to stop
anyway given that other processes can be calling git commands. So we're retrying
a few times before re-raising. To do this we need to clone the command and the current
implementation for that is best-effort.

I do worry about the maintainability of that but we'll see how it goes.

Also, I thought you'd need to clone the task (if it exists) but now I think not;
as long as you don't call done twice on it you should be fine, and you shouldn't
be done'ing a task as part of running a command: that should happen higher up.

pkg/commands/oscommands/cmd_obj.go

@@ -65,6 +65,8 @@ type ICmdObj interface {
 
 	GetCredentialStrategy() CredentialStrategy
 	GetTask() gocui.Task
+
+	Clone() ICmdObj
 }
 
 type CmdObj struct {
@@ -215,3 +217,17 @@ func (self *CmdObj) GetCredentialStrategy() CredentialStrategy {
 func (self *CmdObj) GetTask() gocui.Task {
 	return self.task
 }
+
+func (self *CmdObj) Clone() ICmdObj {
+	clone := &CmdObj{}
+	*clone = *self
+	clone.cmd = cloneCmd(self.cmd)
+	return clone
+}
+
+func cloneCmd(cmd *exec.Cmd) *exec.Cmd {
+	clone := &exec.Cmd{}
+	*clone = *cmd
+
+	return clone
+}