Merge branch 'jk/clone-unborn-confusion' into maint

Merge branch 'jk/clone-unborn-confusion' into maint

"git clone" from a repository with some ref whose HEAD is unborn
did not set the HEAD in the resulting repository correctly, which
has been corrected.
source: <YsdyLS4UFzj0j/wB@coredump.intra.peff.net>

* jk/clone-unborn-confusion:
  clone: move unborn head creation to update_head()
  clone: use remote branch if it matches default HEAD
  clone: propagate empty remote HEAD even with other branches
  clone: drop extra newline from warning message