Turn on sparse-index feature within `git diff-files` command.
Add necessary modifications and test them.

Changes since v1:

1.Add an empty line between the previous test's closing quote
and the start of new test.

2.Use "git add --sparse newdirectory/testfile" instead of 
'git sparse-checkout set' to have staged changes outside 
of the sparse-checkout cone

3.Edit commit message to be more informative

(sorry to send this patch twice ,I forgot to --inreply-to the origin one)

Shuqi Liang (2):
  t1092: add tests for `git diff-files`
  diff-files: integrate with sparse index

 builtin/diff-files.c                     |  4 ++
 t/perf/p2000-sparse-operations.sh        |  2 +
 t/t1092-sparse-checkout-compatibility.sh | 52 ++++++++++++++++++++++++
 3 files changed, 58 insertions(+)


base-commit: a38d39a4c50d1275833aba54c4dbdfce9e2e9ca1
-- 
2.39.0