stringutil: add function to compile stringmatcher pattern into regexp Prepares for adding a revset predicate for "grep --diff". The grep logic needs a regexp object instead of a match function.

py3: fix stringmatcher() to byte-stringify exception message Spotted while writing regexp variant of stringmatcher().

stringutil: extract helper function that splits stringmatcher() pattern

grep: extract main search loop as searcher method Still displayer part is in commands.grep(), the core grep logic is now reusable. I'll revisit the displayer stuff later since it will be another long series.

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.

posix: avoid a leaked file descriptor in a unix domain socket exception case Differential Revision: https://phab.mercurial-scm.org/D9206