fix: texvccheck is not idempotent

The output of texvccheck is not only "checked" but also
rewritten to another form.
(For example $\R$ is transformed to $\mathbb{R}$.)
But the output might not be a valid input for the thexvccheck
part of texvc, like it happened in for the example given in the bug
description.
See also I599c4390da9b8f36d800f379a33ad5ce90f5096c
Bug: 61012
Change-Id: Iae5e350cb78c0e637e574390c586fbdb8dc38496