mdp-tools: spurious "Could not generate diff"

[aelkiss]

When stage-app / deploy-app try to generate diff output, they usually show the diff but then say e.g.:

ERROR: could not invoke
Could not generate diff for application=babel at /htapps/test.babel

We should fix this (and maybe while we're at it, use the colorized git diff output?)

[moseshll]

The "could not invoke" appears to arise from attempting to execute EDITOR on the diff file when EDITOR is not set. That error gets returned to the caller and it incorrectly announces "Could not generate diff..."

A good first step would be to handle EDITOR being unset and issuing a helpful message before bailing out.