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

hgext/rebase.py

@@ -441,8 +441,10 b' class rebaseruntime(object):'
         # Calculate self.obsolete_* sets
         self._handleskippingobsolete()
 
-        rebaseset = destmap.keys()
         if not self.keepf:
+            rebaseset = set(destmap.keys())
+            rebaseset -= set(self.obsolete_with_successor_in_destination)
+            rebaseset -= self.obsolete_with_successor_in_rebase_set
             try:
                 rewriteutil.precheck(self.repo, rebaseset, action=b'rebase')
             except error.Abort as e: