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

tests/test-convert-git.t

@@ -750,7 +750,7 b' test non-tab whitespace .gitmodules'
 test missing .gitmodules
 
   $ git submodule add ../git-repo4 >/dev/null 2>/dev/null
-  $ git checkout HEAD .gitmodules
+  $ git checkout HEAD -- .gitmodules
   $ git rm .gitmodules
   rm '.gitmodules'
   $ git commit -q -m "remove .gitmodules" .gitmodules