git-archive: make tar the default format

As noted by Junio, --format=tar should be assumed if no format
was specified.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/t/t5000-tar-tree.sh b/t/t5000-tar-tree.sh
index b4359df..e223c07 100755
--- a/t/t5000-tar-tree.sh
+++ b/t/t5000-tar-tree.sh
@@ -50,8 +50,16 @@
      git-commit-tree $treeid </dev/null)'
 
 test_expect_success \
+    'git-archive' \
+    'git-archive HEAD >b.tar'
+
+test_expect_success \
     'git-tar-tree' \
-    'git-tar-tree HEAD >b.tar'
+    'git-tar-tree HEAD >b2.tar'
+
+test_expect_success \
+    'git-archive vs. git-tar-tree' \
+    'diff b.tar b2.tar'
 
 test_expect_success \
     'validate file modification time' \