Merge branch 'en/fast-import-avoid-self-replace'

"git fast-import" can be tricked into a replace ref that maps an
object to itself, which is a useless thing to do.

* en/fast-import-avoid-self-replace:
  fast-import: avoid making replace refs point to themselves

diff --cc builtin/fast-import.c
Simple merge

diff --cc t/t9300-fast-import.sh
Simple merge