test-convert-git: remove the read-only attribute of repository files

Git object files are stored read-only in the filesystem. Trying to remove a
read-only file on windows will fail with access denied, so we have to make them
writeable before they can be removed.

tests/test-convert-git.t

@@ -283,9 +283,12 b' damage git repository and convert again'
 
   $ cat > damage.py <<EOF
   > import os
+  > import stat
   > for root, dirs, files in os.walk('git-repo4/.git/objects'):
   >     if files:
   >         path = os.path.join(root, files[0])
+  >         if os.name == 'nt':
+  >             os.chmod(path, stat.S_IWUSR)
   >         os.remove(path)
   >         break
   > EOF