test-convert-git: support older git client (1.4.4.4)

tests/test-convert-git

@@ -32,7 +32,7 b' commit -a -m t1'
 
 # Remove the directory, then try to replace it with a file
 # (issue 754)
-git rm -r d
+git rm -f d/b
 commit -m t2
 echo d > d
 git add d