Support negative indices in getCall

This allows getting calls indexed from the end of the array
e.g. -1 gets the last call, -2 gets the penultimate call etc.

docs/release-source/release/spies.md

@@ -350,6 +350,8 @@ Returns `true` if spy always returned the provided value.
 
 Returns the *nth* [call](#spycall).
 
+If *n* is negative, the *nth* call from the end is returned. For example, `spy.getCall(-1)` returns the last call, and `spy.getCall(-2)` returns the second to last call.
+
 Accessing individual calls helps with more detailed behavior verification when the spy is called more than once.
 
 <div data-example-id="spies-8-spy-call"></div>

lib/sinon/proxy.js

@@ -40,7 +40,11 @@ var proxyApi = {
         return emptyFakes;
     },
 
-    getCall: function getCall(i) {
+    getCall: function getCall(index) {
+        var i = index;
+        if (i < 0) {
+            i += this.callCount;
+        }
         if (i < 0 || i >= this.callCount) {
             return null;
         }

test/spy-test.js

@@ -2086,6 +2086,48 @@ describe("spy", function() {
         });
     });
 
+    describe(".getCall", function() {
+        it("is null for indexes >= length", function() {
+            var spy = createSpy();
+
+            spy();
+
+            assert.isNull(spy.getCall(1));
+            assert.isNull(spy.getCall(2));
+        });
+
+        it("is null for indexes < -length", function() {
+            var spy = createSpy();
+
+            spy();
+
+            assert.isNull(spy.getCall(-2));
+            assert.isNull(spy.getCall(-3));
+        });
+
+        it("is same as last call when passed index -1", function() {
+            var spy = createSpy();
+
+            spy();
+            spy();
+            spy();
+
+            assert.same(spy.getCall(-1).callId, spy.lastCall.callId);
+            assert.same(spy.getCall(-1).spy, spy.lastCall.spy);
+        });
+
+        it("is same as n-1th call when passed index -2", function() {
+            var spy = createSpy();
+
+            spy();
+            spy();
+            spy();
+
+            assert.same(spy.getCall(-2).callId, spy.getCall(1).callId);
+            assert.same(spy.getCall(-2).spy, spy.getCall(1).spy);
+        });
+    });
+
     describe(".lastCall", function() {
         it("is undefined by default", function() {
             var spy = createSpy();