Fix behavior for StripComponents with leading ./

The previous reimplementation of tar --strip-components did not match
how tar treats leading ./ strings in its file headers.
--strip-components should treat the leading . as its own component. The
BOSH agent's usage of the TarballCompressor relies on this behavior, and
the change is causing failures.

fileutil/tarball_compressor.go

@@ -146,7 +146,7 @@ func (c tarballCompressor) DecompressFileToDir(tarballPath string, dir string, o
 		fullName := filepath.Join(dir, filepath.FromSlash(header.Name))
 
 		if options.StripComponents > 0 {
-			components := strings.Split(filepath.Clean(header.Name), string(filepath.Separator))
+			components := strings.Split(header.Name, string(filepath.Separator))
 			if len(components) <= options.StripComponents {
 				continue
 			}

fileutil/tarball_compressor_test.go

@@ -353,9 +353,15 @@ var _ = Describe("tarballCompressor", func() {
 			dstElements, err := pathsInDir(dstDir)
 			Expect(err).ToNot(HaveOccurred())
 
+			// tar --strip-components treats a leading `./` in the file headers as its own component.
+			// So ./dir/some-file becomes dir/some-file with strip-components = 1.
+			// The example tar file in this test contains the leading ./ for each of its files.
 			Expect(dstElements).To(Equal([]string{
 				"./",
-				"double-nested-file",
+				"empty-nested-dir/",
+				"nested-dir/",
+				"nested-dir/double-nested-file",
+				"nested-file",
 			}))
 		})
 	})