Both callers of rewrite_tail immediately compute the new total string
length by adding the (known) length of the existing string plus the
length of the newly appended text.  Unfortunately, callers generally
won't know the length of the new text, as it's printf-formatted.

Since ralloc already computes this length, it makes sense to add it in
and save the caller the effort.  This simplifies both existing callers,
but more importantly, will allow for cheap-appending in the next commit.

v2: The link_uniforms code needs both the old and new length.
    Apply the obvious fix (which sadly makes it less of a cleanup).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> [v1]
Acked-by: José Fonseca <jfonseca@vmware.com> [v1]