rebase: don't call rewriteutil.precheck() with to-be-skipped commits
It's clearly incorrect to call `rewriteutil.precheck()` for commits
that we're not about to rewrite. We haven't noticed yet because the
function doesn't check for divergence, but I'm about to teach it to do
that.
Differential Revision: https://phab.mercurial-scm.org/D10259
#require black test-repo
$ cd $RUNTESTDIR/..
$ black --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`