R1:e591ef4db9aa

Add setup checks for the availability of 'which' and 'diff' binaries

Summary:
Spent an hour or two helping a user figure this out. Make sure I never do that again.

If the webserver is configured with an empty or bogus PATH, binaries like 'which' and 'diff' (and 'git', and 'svn', etc.) may not be available. In most cases, this is fine, because we get an error like "sh: whatever-command not found", which is obvious to diagnose.

In the case of 'diff', we don't get this, because 'diff' is expected to exit…