Set LAST_GOOD_GITSHA for linting only changed files. (#1121)
Set LAST_GOOD_GITSHA for linting only changed files. (#1121)

It seems currently linters.sh execution in prow's presubmit
does check all the files in mixer repository. That would be very
slow.

LAST_GOOD_GITSHA is an environment variable used by linters.sh
in mixer to limit the files to be checked.
istio/old_mixer_repo,istio/old_mixer_repo,ZackButcher/mixer,rkpagadala/mixer,rkpagadala/mixer,ZackButcher/mixer,ZackButcher/mixer,istio/old_mixer_repo,rkpagadala/mixer