pre-upload: Read .presubmitignore files.

Make pre-upload.py look for .presubmitignore files
containing lists of relative-path wildcards, one per line,
matching files that should be excluded from common presubmit
checks.

BUG=chromium:217286
TEST=manual: created a .presubmitignore file and uploaded a
     change to a powerd pref without getting nagged about a
     missing license

Change-Id: I3648e04ed840abd39d8a7fcc53fc7c65d27128ee
Reviewed-on: https://chromium-review.googlesource.com/249882
Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Dan Erat <derat@chromium.org>
Trybot-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
2 files changed