tests: make sure rename pretty print works

Add basic use cases and corner cases tests for
"git diff -M --summary/stat".

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'ap/maint-diff-rename-avoid-overlap' into next

* ap/maint-diff-rename-avoid-overlap:
  tests: make sure rename pretty print works