commit: raise when given bad data to extract the signature

This lets the caller distinguish between a missing object, the wrong
kind of object and unsigned commits.

Author: carlosmn

ext/rugged/rugged_commit.c

@@ -647,7 +647,7 @@ static VALUE rb_git_commit_extract_signature(int argc, VALUE *argv, VALUE self)
 		git_buf_free(&signed_data);
 	}
 
-	if (error == GIT_ENOTFOUND) {
+	if (error == GIT_ENOTFOUND && giterr_last()->klass == GITERR_OBJECT ) {
 		ret_arr = rb_ary_new3(2, Qnil, Qnil);
 	} else {
 		rugged_exception_check(error);

test/commit_test.rb

@@ -273,6 +273,12 @@ def test_extract_signature
     assert_equal exp_signed_data, signed_data
 
     assert_equal [nil, nil], Rugged::Commit.extract_signature(@repo, "8496071c1b46c854b31185ea97743be6a8774479")
+
+    # Ask for a tree
+    assert_raises(Rugged::InvalidError) { Rugged::Commit.extract_signature(@repo, "181037049a54a1eb5fab404658a3a250b44335d7") }
+
+    # Ask for a non-existent object
+    assert_raises(Rugged::OdbError) { Rugged::Commit.extract_signature(@repo, "181037049a54a1eb5fab404658a3a250b44335d8") }
   end
 end