revlog: rename variable in `rewrite_sidedata` to match other code

Authored by marmoute.


revlog: rename variable in rewrite_sidedata to match other code

Let's call the index file object and ifh and the data file object dfh as the
rest of the revlog code. This will make future change clearer.

Differential Revision: https://phab.mercurial-scm.org/D10606