AffectedFiles carefully added a slash to dir_with_slash, but then called
normpath - which trims trailing path separators. This meant that
ui/views/PRESUBMIT.py would analyze files in ui\views_content_client,
leading to spooky action at a distance. This was noticed when this
command triggered a presubmit error:

    git cl presubmit "--files=ui/views_content_client/*.cc;ui/views/readme.md"

when running presubmit on either file individually found nothing.

This change was tested by getting CheckChangeLintsClean to print the
files that it was asked to analyze, making the behavior quite obvious.

This bug appears to have existed for about thirteen years, but only
triggers in rare cases, and even then the incorrect behavior was almost
impossible to notice.

One of the tests was inadvertently testing the broken AffectedFiles
behavior and another seemed to be requiring it to handle '.'
correctly which I'm not sure we want to support.

Bug: 1309977
Change-Id: Ibdc39981d69664b03448acb228d4ab05b49436f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3632034
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>