tar: fix a bug where autodetection messes up -z on extract

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/tar.tests

@@ -154,6 +154,20 @@ dr-xr-x--- input_dir
 SKIP=
 }
 
+# Had a bug where on extrace autodetect first "switched off" -z
+# and then failed to recognize .tgz extension
+testing "tar extract tgz" "\
+dd count=1 bs=1M if=/dev/zero of=F0 2>/dev/null
+tar -czf F0.tgz F0
+rm F0
+tar -xzvf F0.tgz && echo Ok
+rm F0 || echo BAD
+" "\
+F0
+Ok
+" \
+"" ""
+
 
 cd .. && rm -rf tar.tempdir || exit 1