Merge branch 'jk/maint-tformat-with-z'

Merge branch 'jk/maint-tformat-with-z'

"log -z --pretty=tformat:..." does not terminate each record with NUL
and this is a beginning of an attempt to fix it.  It still is not right
but the patch does not make externally observable behaviour worse.

By Jan Krüger (1) and Junio C Hamano (1)
* jk/maint-tformat-with-z:
  log-tree: the previous one is still not quite right
  log-tree: use custom line terminator in line termination mode