DO not invoke git config because HOMEBREW_NO_ANALYTICS, otherwise
it will disable analytics for everyone when running `brew update`
in following manners:
* `brew update` will set HOMEBREW_NO_ANALYTICS because the absence of
`homebrew.analyticsmessage`
* `brew update-report` will set `homebrew.analyticsdisabled` because of
HOMEBREW_NO_ANALYTICS.
Also reduce file IO.
7aaaf9d8db