Merge branch 'jc/grep-pcre-loose-ends'

"git log -F -E --grep='<ere>'" failed to use the given <ere>
pattern as extended regular expression, and instead looked for the
string literally.  The early part of this series is a fix for it;
the latter part teaches log to respect the grep.* configuration.

* jc/grep-pcre-loose-ends:
  log: honor grep.* configuration
  log --grep: accept --basic-regexp and --perl-regexp
  log --grep: use the same helper to set -E/-F options as "git grep"
  revisions: initialize revs->grep_filter using grep_init()
  grep: move pattern-type bits support to top-level grep.[ch]
  grep: move the configuration parsing logic to grep.[ch]
  builtin/grep.c: make configuration callback more reusable