disable git diff colors in git lb-push

with

[color]
  diff = always # keep colors alive when piping through grep/sed/head or into `less -R`

in the $(HOME)/.gitconfig, calling git lb-push fails as outlined here.

This can be repaired (other than, of cause, on the user side) by explicitly adding --no-color in the git diff call in git-lb-push. It appears color settings for git log and git status do not interfere with the git lb-* commands