use mock-git to increase coverage

.gitignore

@@ -1 +1,2 @@
 /node_modules/
+/coverage/

package.json

@@ -33,10 +33,11 @@
     "istanbul": "^0.3.17",
     "jscs": "^2.0.0",
     "jshint": "^2.8.0",
-    "mocha": "*"
+    "mocha": "*",
+    "mock-git": "^1.0.2"
   },
   "scripts": {
-    "coverage": "istanbul cover _mocha -- -R spec && rm -rf ./coverage",
+    "coverage": "istanbul cover _mocha -- -R spec",
     "lint": "jshint *.js test --exclude node_modules && jscs *.js test",
     "test": "npm run-script lint && mocha"
   },

test/test.js

@@ -1,12 +1,27 @@
 'use strict';
 var assert = require('assert');
 var gitTails = require('../');
+var mockGit = require('mock-git');
+var unmock;
 
 it('should throw if no callback', function() {
   assert.throws(gitTails);
 });
 
+it('should callback with error', function(done) {
+  mockGit('throw new Error("Suck it")', 'rev-list')
+    .then(function(_unmock_) {
+      unmock = _unmock_;
+
+      gitTails(function(err) {
+        assert.ok(err);
+        done();
+      });
+    });
+});
+
 it('should get the first commit', function(done) {
+  unmock();
   gitTails(function(err, data) {
     assert.deepEqual(data, ['9594829e694c5284ab6ef3816ded02ca104fee59']);
     done();