revlog: simplify entry update logic in `rewrite_sidedata`
We don't need to rebuild the full entry. So lets just gather the data we need
to pass to `replace_sidedata_info`.
Differential Revision: https://phab.mercurial-scm.org/D10642
#require black test-repo
Black needs the real USERPROFILE in order to run on Windows
#if msys
$ USERPROFILE="$REALUSERPROFILE"
$ export USERPROFILE
#endif
$ cd $RUNTESTDIR/..
$ black --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`