Default "environment.append-paths" to include likely paths

Summary:
A few more of these issues have cropped up recently. Basically:

• Webservers often (by default, I guess?) have a different or nonexistent $PATH.
• Users have a hard time figuring this out, since it's not obvious that the webserver might have a different configuration than the CLI, and they can run "git" and such themselves fine, and they don't normally use SetEnv or similar in webserver config.

I've been pursuing one prong of attack here (better detection and more tailored errors); this is a second prong (try to just guess the configuration correctly).

In 99% of cases, the binaries in question are in one of these three places, so just make them the default appended paths. If users have wacky configs they can override the setting.

Test Plan: Viewed config locally.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6039