t0107: Use `tar -z` for gzip'ed archives

Some tar(1) versions do not support auto detection of the compression
type. Explicitly specify "-z" to decompress a ".tar.gz" archive.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

tests/t0107-snapshot.sh

@@ -25,7 +25,7 @@ test_expect_success 'verify gzip format' '
 
 test_expect_success 'untar' '
 	rm -rf master &&
-	tar -xf master.tar.gz
+	tar -xzf master.tar.gz
 '
 
 test_expect_success 'count files' '