Handle non-zero exit code from git diff

The check for a changed repo in setup_develop() 'git diff --exit-code'
returns a status of 1 when the repo has changes; trap that so errexit
does not abort the script.

Bug-Id: 1285780
Change-Id: Ic97e68348f46245b271567893b447fcedbd7bd6e
Merge "Handle non-zero exit code from git diff"