Merge branch 'sg/config-name-only'

Merge branch 'sg/config-name-only'

"git config --list" output was hard to parse when values consist of
multiple lines.  "--name-only" option is added to help this.

* sg/config-name-only:
  get_urlmatch: avoid useless strbuf write
  format_config: simplify buffer handling
  format_config: don't init strbuf
  config: restructure format_config() for better control flow
  completion: list variable names reliably with 'git config --name-only'
  config: add '--name-only' option to list only variable names