This removes all svn features from the release scripts.

This also fixes a bug in commit position retrieval.

BUG=chromium:410721
LOG=n
R=tandrii@chromium.org
TBR=tandrii@chromium.org
TEST=script_test.py

Review URL: https://codereview.chromium.org/716153002

Cr-Commit-Position: refs/heads/master@{#25307}