Warn when 'git-http-backend' is not on the PATH.

Summary:
Currently if 'git-http-backend' is not on the PATH, there is no visible message to the user other than "info/refs: is this a valid git repository?" when trying to clone.  This adds a setup check so that if there are any Git repositories in use, it will check for the existance of the "git-http-backend" binary in the PATH.

I believe this is shipped by default alongside the git package on most distros, but in some (such as OpenSUSE), this binary isn't on the PATH by default.

Test Plan: Removed `/usr/lib/git` from my `environment.append-paths` and saw the message appear.  Added it back and the message went away.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4050

Differential Revision: https://secure.phabricator.com/D7488