avoid duplicate chdir, fixes SD-1442
avoid duplicate chdir, fixes SD-1442

test plan:
1. lint_commit_message should run and pass on this commit
2. it should lint a plugin commit if you set GERGICH_GIT_PATH and
   GERRIT_REFSPEC and not blow up on linting failures

Change-Id: I7349eb0885038ac11660b95a7e528735048be2bc
Reviewed-on: https://gerrit.instructure.com/88104
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>
QA-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>

sfu/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,dgynn/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,dgynn/canvas-lms,dgynn/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,dgynn/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms