watch: Avoid duplicate files in watchlist
watch: Avoid duplicate files in watchlist

Since the 'files' section in nyc's index is fully resolved, but the
'externalIds' is not, you can end up in a case where a file is in both
sets, which means that it'll run the test suite twice when that file is
changed.

Fix #577

tapjs/node-tap,evanlucas/node-tap,isaacs/node-tap,tapjs/node-tap,evanlucas/node-tap,isaacs/node-tap