refs.c: log_ref_write should try to return meaningful errno

refs.c: log_ref_write should try to return meaningful errno

Making errno from write_ref_sha1() meaningful, which should fix

* a bug in "git checkout -b" where it prints strerror(errno)
  despite errno possibly being zero or clobbered

* a bug in "git fetch"'s s_update_ref, which trusts the result of an
  errno == ENOTDIR check to detect D/F conflicts

Signed-off-by: Ronnie Sahlberg <redacted>
Signed-off-by: Junio C Hamano <redacted>
Acked-by: Michael Haggerty <redacted>