Only test what matters: error codes are unique (apart from success)

The tests failed when the upstream library rearranged its constants; but
the value is not an API feature, so we shouldn't depend on it but instead
test that error conditions return the right constant by triggering those
errors.

t/git.t

@@ -163,37 +163,25 @@ sub test_version {
 
 
 sub test_constants {
-    my $code = 0;
-    is(Git2::GIT_SUCCESS, $code--, "GIT_SUCCESS is ok");
-    is(Git2::GIT_ERROR, $code--, "GIT_ERROR is ok");
-    is(Git2::GIT_ENOTOID, $code--, "GIT_ENOTOID is ok");
-	is(Git2::GIT_ENOTFOUND, $code--, "GIT_ENOTFOUND is ok");
-	is(Git2::GIT_ENOMEM, $code--, "GIT_ENOMEM is ok");
-	is(Git2::GIT_EOSERR, $code--, "GIT_EOSERR is ok");
-	is(Git2::GIT_EOBJTYPE, $code--, "GIT_EOBJTYPE is ok");
-	is(Git2::GIT_EOBJCORRUPTED, $code--, "GIT_EOBJCORRUPTED is ok");
-	is(Git2::GIT_ENOTAREPO, $code--, "GIT_ENOTAREPO is ok");
-	is(Git2::GIT_EINVALIDTYPE, $code--, "GIT_EINVALIDTYPE is ok");
-	is(Git2::GIT_EMISSINGOBJDATA, $code--, "GIT_EMISSINGOBJDATA is ok");
-	is(Git2::GIT_EPACKCORRUPTED, $code--, "GIT_EPACKCORRUPTED is ok");
-	is(Git2::GIT_EFLOCKFAIL, $code--, "GIT_EFLOCKFAIL is ok");
-	is(Git2::GIT_EZLIB, $code--, "GIT_EZLIB is ok");
-	is(Git2::GIT_EBUSY, $code--, "GIT_EBUSY is ok");
-	is(Git2::GIT_EBAREINDEX, $code--, "GIT_EBAREINDEX is ok");
-	is(Git2::GIT_EINVALIDREFNAME, $code--, "GIT_EINVALIDREFNAME is ok");
-	is(Git2::GIT_EREFCORRUPTED , $code--, "GIT_EREFCORRUPTED  is ok");
-	is(Git2::GIT_ETOONESTEDSYMREF, $code--, "GIT_ETOONESTEDSYMREF is ok");
-	is(Git2::GIT_EPACKEDREFSCORRUPTED, $code--, "GIT_EPACKEDREFSCORRUPTED is ok");
-	is(Git2::GIT_EINVALIDPATH, $code--, "GIT_EINVALIDPATH is ok");
-	is(Git2::GIT_EREVWALKOVER, $code--, "GIT_EREVWALKOVER is ok");
-	is(Git2::GIT_EINVALIDREFSTATE, $code--, "GIT_EINVALIDREFSTATE is ok");
-	is(Git2::GIT_ENOTIMPLEMENTED, $code--, "GIT_ENOTIMPLEMENTED is ok");
-	is(Git2::GIT_EEXISTS, $code--, "GIT_EEXISTS is ok");
-	is(Git2::GIT_EOVERFLOW, $code--, "GIT_EOVERFLOW is ok");
-	is(Git2::GIT_ENOTNUM, $code--, "GIT_ENOTNUM is ok");
-
-
-	$code = -2;
+
+    my @constants =
+	qw( ERROR ENOTOID ENOTFOUND ENOMEM EOSERR EOBJTYPE
+	    EOBJCORRUPTED ENOTAREPO EINVALIDTYPE EMISSINGOBJDATA
+	    EPACKCORRUPTED EFLOCKFAIL EZLIB EBUSY EBAREINDEX
+	    EINVALIDREFNAME EREFCORRUPTED ETOONESTEDSYMREF
+	    EPACKEDREFSCORRUPTED EINVALIDPATH EREVWALKOVER
+	    EINVALIDREFSTATE ENOTIMPLEMENTED EEXISTS EOVERFLOW ENOTNUM
+	    );
+
+    is(Git2::GIT_SUCCESS, 0, "GIT_SUCCESS (0) is OK");
+    no strict 'refs';
+    my %seen;
+    for my $constant ( @constants ) {
+	my $val = &{"Git2::GIT_$constant"};
+	ok(($val<0) && !$seen{$val}++, "$constant is <0 and unique");
+    }
+
+    my $code = -2;
 	is(Git2::GIT_OBJ_ANY, $code++, "GIT_OBJ_ANY is ok");
 	is(Git2::GIT_OBJ_BAD, $code++, "GIT_OBJ_BAD is ok");
 	is(Git2::GIT_OBJ__EXT1, $code++, "GIT_OBJ__EXT1 is ok");