Merge branch 'mm/diff-no-index-find-object'

"git diff --no-index --find-object=<object-name>" outside a
repository of course wouldn't be able to find the object and died
while parsing the command line, which is made to die in a bit more
user-friendly way.

* mm/diff-no-index-find-object:
  diff: fix crash with --find-object outside repository

diff --cc diff.c
Simple merge

diff --cc t/t4053-diff-no-index.sh
Simple merge