Check that git is installed when instantiating a git resolver

lib/core/resolvers/GitResolver.js

@@ -7,9 +7,22 @@ var rimraf = require('rimraf');
 var mout = require('mout');
 var Resolver = require('./Resolver');
 var createError = require('../../util/createError');
+var which = require('which');
+
+var gitChecked = false;
 
 var GitResolver = function (source, options) {
     Resolver.call(this, source, options);
+
+    if (!gitChecked) {
+        try {
+            which.sync('git');
+        } catch (ex) {
+            throw new Error('git is not installed or not in the PATH');
+        }
+    }
+
+    gitChecked = true;
 };
 
 util.inherits(GitResolver, Resolver);