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