Refactoring: Unify command mocks in v8 roll tests.

This unifies all mocks for shell commands (like git, svn,
gclient). The command name is now explicitly stated in the
test expectations, which allows to correctly test the few
commands that are not git.

TEST=./script_test.py
TBR=jarin@chromium.org

Review URL: https://codereview.chromium.org/528973002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23611 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix chromium update in chromium roll script.

Needs an extra 'git pull' after a gclient sync, because the solution is
unmanaged now.

TBR=jarin@chromium.org,hinoka@chromium.org

Review URL: https://codereview.chromium.org/529883003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23612 ce2b1a6d-e550-0410-aec6-3dcde31c8c00