[tools/vim] Only run PRESUBMIT unittests for changes that touch YCM config.

BUG=554183

Review URL: https://codereview.chromium.org/1468513002

Cr-Original-Commit-Position: refs/heads/master@{#360985}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 167e1ea5c14c634c118c5155b5ab9daaab2902ee