Warn users when cloning apps that were build from git branch with uncommitted changes.

[sangee2004]

acorn version - v0.9.0-rc1-29-g28f2b006+28f2b006

Steps to reproduce the problem:

1. Deploy app from a git directory were changes are made to Acornfile and few other files but not committed yet.
2. Clone this app using `acorn dev --clone -n

Cloned directory has the changes done to Acornfile but doesnot have changes made to other files.

This is a confusing behavior.

From @cjellick I am considering more guard rails and warnings…like we could in theory tell the user “You are about to use —clone on an image that was built with uncommitted changes. Do you want to proceed? Y/n”