Racy GIT (part #3)

Commit 29e4d36357 fixed the
underlying update-index races but git-commit was not careful
enough to preserve the index file timestamp when copying the
index file.  This caused t3402 test to occasionally fail.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-commit.sh

@@ -29,7 +29,7 @@ THIS_INDEX="$GIT_DIR/index"
 NEXT_INDEX="$GIT_DIR/next-index$$"
 rm -f "$NEXT_INDEX"
 save_index () {
-	cp "$THIS_INDEX" "$NEXT_INDEX"
+	cp -p "$THIS_INDEX" "$NEXT_INDEX"
 }
 
 report () {