Retrieve current version from trunk branch in push-to-trunk.

- This moves retrieving and incrementing the version before creating the change log
- Before the prepare push commit will be deprecated (follow up CL), the script deals with 3 build levels. X: the current build level on the last trunk push. X + 1: the build level for this trunk push. X + 2: the build level of the new version file on bleeding edge (to be deprecated).

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/203773013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20071 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Prepare push to trunk.  Now working on version 3.25.20.

R=yangguo@chromium.org
TBR=yangguo@chromium.org

Review URL: https://codereview.chromium.org/203193014

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20072 ce2b1a6d-e550-0410-aec6-3dcde31c8c00