envsetup: Rework cmgerrit() - A Git wrapper

Usage:
    cmgerrit COMMAND [OPTIONS] CHANGE_ID[/PATCH_SET][{@|^|~|:}ARG] [-- ARGS]

Any Git command that with refname as its parameter should work. Basically,
this works as:
    git fetch PROJECT_URL refs/changes/HASH/CHANGE_ID/PATCH_SET \
    && git COMMAND OPTIONS FETCH_HEAD{@|^|~|:}ARG -- ARGS

Examples:
    cmgerrit pull 1234
    cmgerrit push my_patch:gingerbread
    cmgerrit checkout -B patch_1234 1234/5
    cmgerrit branch fix_xyz 1234/5 && git merge fix_xyz
    cmgerrit show 1234:file
    cmgerrit checkout 1234 -- file1 file2 && git diff --cached

Change-Id: Id85d58db79c010428e3aed0cc0f22e04f0963ac0