Merge branch 'jk/commit-author-parsing'

Merge branch 'jk/commit-author-parsing'

Code clean-up.

* jk/commit-author-parsing:
  determine_author_info(): copy getenv output
  determine_author_info(): reuse parsing functions
  date: use strbufs in date-formatting functions
  record_author_date(): use find_commit_header()
  record_author_date(): fix memory leak on malformed commit
  commit: provide a function to find a header in a buffer