Fixed exception when adding new branch specs to config

Author: JakeGinnivan

GitVersionCore.Tests/ConfigProviderTests.cs

@@ -88,6 +88,20 @@ public void OverwritesDefaultsWithProvidedConfig()
         config.Branches["develop"].Tag.ShouldBe("dev");
     }
 
+    [Test]
+    public void CanProvideConfigForNewBranch()
+    {
+        const string text = @"
+next-version: 2.0.0
+branches:
+    bug[/-]:
+        tag: bugfix";
+        SetupConfigFileContent(text);
+        var config = ConfigurationProvider.Provide(gitDirectory, fileSystem);
+        
+        config.Branches["bug[/-]"].Tag.ShouldBe("bugfix");
+    }
+
     [Test]
     [MethodImpl(MethodImplOptions.NoInlining)]
     public void CanWriteOutEffectiveConfiguration()

GitVersionCore/Configuration/Config.cs

@@ -72,7 +72,13 @@ public Dictionary<string, BranchConfig> Branches
             }
             set
             {
-                value.ToList().ForEach(_ => branches[_.Key] = MergeObjects(branches[_.Key],  _.Value));
+                value.ToList().ForEach(_ =>
+                {
+                    if (!branches.ContainsKey(_.Key))
+                        branches.Add(_.Key, new BranchConfig());
+
+                    branches[_.Key] = MergeObjects(branches[_.Key], _.Value);
+                });
             }
         }