tests: suppress "Checked out 1 paths of <hash>" from modern git
test-convert-git.t is failiing since git commit 0f086e6dca (checkout:
print something when checking out paths, 2018-11-13). Suppress the new
output by disambiguating the arguments with a "--" separator.
Differential Revision: https://phab.mercurial-scm.org/D5625