archive/tar: fix round-trip attributes

The issue was identified while
working with round trip FileInfo of the headers of hardlinks. Also,
additional test cases for hard link handling.
(review carried over from http://golang.org/cl/165860043)

Fixes #9027

Change-Id: I9e3a724c8de72eb1b0fbe0751a7b488894911b76
Reviewed-on: https://go-review.googlesource.com/6790
Reviewed-by: Russ Cox <rsc@golang.org>

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

archive/tar/writer_test.go

@@ -147,6 +147,44 @@ var writerTests = []*writerTest{
 			},
 		},
 	},
+	// This file was produced using gnu tar 1.26
+	// echo "Slartibartfast" > file.txt
+	// ln file.txt hard.txt
+	// tar -b 1 --format=ustar -c -f hardlink.tar file.txt hard.txt
+	{
+		file: "testdata/hardlink.tar",
+		entries: []*writerTestEntry{
+			{
+				header: &Header{
+					Name:     "file.txt",
+					Mode:     0644,
+					Uid:      1000,
+					Gid:      100,
+					Size:     15,
+					ModTime:  time.Unix(1425484303, 0),
+					Typeflag: '0',
+					Uname:    "vbatts",
+					Gname:    "users",
+				},
+				contents: "Slartibartfast\n",
+			},
+			{
+				header: &Header{
+					Name:     "hard.txt",
+					Mode:     0644,
+					Uid:      1000,
+					Gid:      100,
+					Size:     0,
+					ModTime:  time.Unix(1425484303, 0),
+					Typeflag: '1',
+					Linkname: "file.txt",
+					Uname:    "vbatts",
+					Gname:    "users",
+				},
+				// no contents
+			},
+		},
+	},
 }
 
 // Render byte array in a two-character hexadecimal string, spaced for easy visual inspection.