Fix progress check in auto-roller.

Fix progress check in auto-roller.

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

Committed: https://crrev.com/f96e226011b1cd5e1cb959a36d9152de364be1ec
Cr-Commit-Position: refs/heads/master@{#27021}

[Michael Achenbach, 2015-03-05 13:36:22]

machenbach@chromium.org changed reviewers:
+ hablich@chromium.org, tandrii@chromium.org

[Michael Achenbach, 2015-03-05 13:39:24]

PTAL

[Michael Achenbach, 2015-03-05 14:04:28]

The CQ bit was checked by machenbach@chromium.org

[commit-bot: I haz the power, 2015-03-05 14:04:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/979133002/1

[commit-bot: I haz the power, 2015-03-05 14:05:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-05 14:05:20]

Patchset 1 (id:??) landed as
https://crrev.com/f96e226011b1cd5e1cb959a36d9152de364be1ec
Cr-Commit-Position: refs/heads/master@{#27021}

[tandrii(chromium), 2015-03-09 10:46:03]

lgtm