Using a git range check for checking progress is wrong when
the last rolled revision and the revision candidate are on
different branches. The range A..B will always show the
commits from the merge-base of A and B until B.

Better compare the tags of the last rolled revision and the
candidate. The candidate's version must be strictly greater
than what's in chromium.

TBR=tandrii@chromium.org
NOTRY=true
TEST=./script_test.py

Review URL: https://codereview.chromium.org/979133002

Cr-Commit-Position: refs/heads/master@{#27021}