tests: fix test-convert-git to work w/ "git pull" requiring strategy A recent change to git (031e2f7ae195) made it an error to not specify a strategy (`--rebase`, `--no-rebase`, `--ff-only`), instead of just the warning it was previously. As far as I can tell, `--no-rebase` is the behavior we were getting before, and the only one that makes the test work. Differential Revision: https://phab.mercurial-scm.org/D11714