git-reset: do not be confused if there is nothing to reset

The purpose of the function update_index_from_diff() (which is the
callback function we give do_diff_cache()) is to update those index
entries which differ from the given commit.

Since do_diff_cache() plays games with the in-memory index, this function
discarded the cache and reread it.

Then, back in the function read_from_tree() we wrote the index.

Of course, this broke down when there were no changes and
update_index_from_diff() was not called, and therefore the mangled index
was not discarded.

The solution is to move the index writing into the function
update_index_from_diff().

Noticed by Björn Steinbrink.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7102-reset.sh

@@ -402,4 +402,11 @@ test_expect_success 'test resetting the index at give paths' '
 
 '
 
+test_expect_success 'resetting an unmodified path is a no-op' '
+	git reset --hard &&
+	git reset -- file1 &&
+	git diff-files --exit-code &&
+	git diff-index --cached --exit-code HEAD
+'
+
 test_done