diff-index and diff-files can get confused in corner cases when an indexed
blob turns into something else in the work tree. This patch adds tests to
expose such breakages.
The test is classified under t2XXX series instead of t4XXX series, because
the ultimate objective is to fix "add -u" (and "commit -a" that shares the
same issue).
Signed-off-by: Junio C Hamano <gitster@pobox.com>
