Merge branch 'bf/fetch-set-head-config'

Merge branch 'bf/fetch-set-head-config'

"git fetch" honors "remote.<remote>.followRemoteHEAD" settings to
tweak the remote-tracking HEAD in "refs/remotes/<remote>/HEAD".

* bf/fetch-set-head-config:
  remote set-head: set followRemoteHEAD to "warn" if "always"
  fetch set_head: add warn-if-not-$branch option
  fetch set_head: move warn advice into advise_if_enabled
  fetch: add configuration for set_head behaviour