Merge branch 'mb/fast-import-delete-root'

An attempt to remove the entire tree in the "git fast-import" input
stream caused it to misbehave.

* mb/fast-import-delete-root:
  fast-import: fix segfault in store_tree()
  t9300: test filedelete command

diff --cc fast-import.c
Simple merge

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