A helper function that takes the contents of a commit object and
finds its subject line did not ignore leading blank lines, as is
commonly done by other codepaths.  Make it ignore leading blank
lines to match.

* js/find-commit-subject-ignore-leading-blanks:
  reset --hard: skip blank lines when reporting the commit subject
  sequencer: use skip_blank_lines() to find the commit subject
  commit -C: skip blank lines at the beginning of the message
  commit.c: make find_commit_subject() more robust
  pretty: make the skip_blank_lines() function public