Merge branch 'jc/grep-pcre-loose-ends' (early part) into maint

"git log -F -E --grep='<ere>'" failed to use the given <ere>
pattern as extended regular expression, and instead looked for the
string literally.

* 'jc/grep-pcre-loose-ends' (early part):
  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