Merge branch 'jc/filter-commit'

Merge branch 'jc/filter-commit'

* jc/filter-commit:
  git log: Unify header_filter and message_filter into one.
  Update grep internal for grepping only in head/body
  git-log --author and --committer are not left-anchored by default
  rev-list: fix segfault with --{author,committer,grep}
  revision traversal: --author, --committer, and --grep.
  revision traversal: prepare for commit log match.
  builtin-grep: make pieces of it available as library.