Git::clone: correct onlyif checks

The exit status of 'find' is 0 if no problems were encountered executing the
search. The number of results have no bearing on it. '-print -quit' (both GNU
extensions) will cause find to print the first match and terminate the search;
grep '' checks if there was a match at all.

Change-Id: If6efa9bc17e2a78744cb2ad9a3c42fb962348834
Merge "Git::clone: correct onlyif checks" into production