Add git_attr methods and ensure prototype exists

Author: tbranyen

generate/descriptor.json

@@ -1,6 +1,31 @@
 {
   "attr": {
-    "cType": null
+    "cType": null,
+
+    "functions": {
+      "git_attr_get": {
+        "ignore": false
+      },
+
+      "git_attr_get_many": {
+        "ignore": false
+      },
+
+      "git_attr_foreach": {
+        "ignore": true,
+        "return": {
+          "isErrorCode": true
+        }
+      },
+
+      "git_attr_cache_flush": {
+        "ignore": false
+      },
+
+      "git_attr_add_macro": {
+        "ignore": false
+      }
+    }
   },
 
   "blame": {

lib/attr.js

@@ -0,0 +1,5 @@
+var NodeGit = require("../");
+
+var Attr = NodeGit.Attr;
+
+module.exports = Attr;

lib/nodegit.js

@@ -24,7 +24,7 @@ descriptors.forEach(function(descriptor) {
     var original = null;
 
     // Special case when you have a prototype method.
-    if (asyncFunc.isPrototypeMethod) {
+    if (asyncFunc.isPrototypeMethod && Ctor.prototype) {
       original = Ctor.prototype[asyncFunc.jsFunctionName];
       Ctor.prototype[asyncFunc.jsFunctionName] = promisify(original);
     }