GN: restore gn.py partly as fetch-gn

Same as before, except it explicitly only fetches GN instead of acting
like a proxy for it.  I found myself always typing 'gn' anyway.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2167353002

Review-Url: https://codereview.chromium.org/2167353002
1 file changed