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
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