Merge branch 'mh/fetch-follow-remote-head-config' into jch

Merge branch 'mh/fetch-follow-remote-head-config' into jch

The `fetch.followRemoteHEAD` configuration variable has been added to
provide a default for the per-remote `remote.<name>.followRemoteHEAD`
setting.

* mh/fetch-follow-remote-head-config:
  fetch: fixup a misaligned comment
  fetch: add configuration variable fetch.followRemoteHEAD
  fetch: refactor do_fetch handling of followRemoteHEAD
  fetch: return 0 on known git_fetch_config
  fetch: rename function report_set_head
  t5510: cleanup remote in followRemoteHEAD dangling ref test
  doc: explain fetchRemoteHEADWarn advice
  fetch: fixup set_head advice for warn-if-not-branch