scmutil: move walkchangerevs() from cmdutil It's no longer a command-level function, but a pure helper to walk revisions in a windowed way. This change will help eliminate reverse dependency of revset.py -> grep.py -> cmdutil.py in future patches.

grep: extract public function to register file to be skipped The main grep loop will be extracted to a searcher method, but this skipping condition depends on the result of display() function.