A Git fetch refspec normally looks like 'refs/foo/bar:refs/qux/baz'.
In English that means: resolve 'refs/foo/bar' on the remote server,
fetch the object it points to, and create a local reference
'refs/qux/baz' that points to that thing.

However, since Git 1.8.3 (released in 2013) it is also allowed to
write '$COMMIT_SHA:refs/qux/baz'. What that does is: fetch object
$COMMIT_SHA from the server, and create a local 'refs/qux/baz' that
points to that thing.

This commit changes the refspec used for "persistent ref" CI fetches.
We already know what commit SHA persistent refs point to so there is
no need to have Git resolve the persistent ref again during a CI
build.