pre-upload: Fix comparison between absolute and relative path.

In _check_for_uprev(), affected_paths, set of relative paths the CL
affects was compared with a set of full paths of affected ebuild files.

BUG=chromium:440138
TEST=Unit tests: check that new unit test fails before change, pass after.
Check that CL that failed verification (cl/233233) now passes.

Change-Id: I74ed0ab8d75d1ccab6d3e2e11464fecaf0b4c317
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/233912
2 files changed