Creating branch can fail silently on name conflict

[StanleyGoldman]

The following actions fail silently from the user's perspective.

1. If branch foobar exists, attempting to create a branch named foobar
2. If branch foo/bar exists, attempting to create a branch named foo

170629-17:02:31 TRACE [23] <TaskManager>                       Exception on exclusive thread: 2 git branch
fatal: A branch named 'asdf' already exists.
  at GitHub.Unity.ProcessTask`1+<RunWithReturn>c__AnonStorey0[System.String].<>m__0 () [0x00116] in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\ProcessTask.cs:295 
  at GitHub.Unity.ProcessWrapper.Run () [0x002fc] in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\ProcessTask.cs:164 
  at GitHub.Unity.ProcessTask`1[T].RunWithReturn (Boolean success) [0x00063] in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\ProcessTask.cs:305 
  at GitHub.Unity.TaskBase`1[TResult].<TaskBase>m__0 () [0x00043] in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\TaskBase.cs:406 
  at System.Threading.Tasks.Task`1[System.String].InvokeFuture (System.Object futureAsObj) [0x00000] in <filename unknown>:0 
  at System.Threading.Tasks.Task.InnerInvoke () [0x00000] in <filename unknown>:0 
  at System.Threading.Tasks.Task.Execute () [0x00000] in <filename unknown>:0 
   at GitHub.Unity.LogFacade.Trace(System.Exception ex, System.String message) in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Logging\LogFacade.cs:line 91
   at GitHub.Unity.TaskManager+<ScheduleExclusive>c__AnonStorey1`1[[GitHub.Unity.TaskBase, GitHub.Api, Version=0.12.0.0, Culture=neutral, PublicKeyToken=null]].<>m__0(System.Threading.Tasks.Task tt) in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\TaskManager.cs:line 130
   at System.Threading.Tasks.Task+<>c__DisplayClass6.<ContinueWith>b__5(System.Object obj)
   at System.Threading.Tasks.Task.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   at System.Threading.Tasks.Task.ExecutionContextCallback(System.Object obj)
   at System.Security.SecurityContext.Run(System.Security.SecurityContext securityContext, System.Threading.ContextCallback callback, System.Object state) in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Security/SecurityContext.cs:line 150
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state) in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Threading/ExecutionContext.cs:line 150
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef currentTaskSlot)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean bPreventDoubleExecution)
   at System.Threading.Tasks.TaskScheduler.TryExecuteTask(System.Threading.Tasks.Task task)
   at GitHub.Unity.ConcurrentExclusiveInterleave+ConcurrentExclusiveTaskScheduler.ExecuteTask(System.Threading.Tasks.Task task) in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\ConcurrentExclusiveInterleave.cs:line 398
   at GitHub.Unity.ConcurrentExclusiveInterleave.ExecuteConcurrentTask(System.Threading.Tasks.Task task) in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\ConcurrentExclusiveInterleave.cs:line 214
   at System.Threading.Tasks.Parallel+<>c__DisplayClass32`2[[System.Threading.Tasks.Task, System.Threading, Version=1.0.2856.102, Culture=neutral, PublicKeyToken=31bf3856ad364e35],[System.Object, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].<PartitionerForEachWorker>b__30()
   at System.Threading.Tasks.Task.InnerInvoke()
   at System.Threading.Tasks.Task.InnerInvokeWithArg(System.Threading.Tasks.Task childTask)
   at System.Threading.Tasks.Task+<>c__DisplayClass3.<ExecuteSelfReplicating>b__2(System.Object )
   at System.Threading.Tasks.Task.ExecuteSelfReplicating(System.Threading.Tasks.Task root)
   at System.Threading.Tasks.Task.ExecutionContextCallback(System.Object obj)
   at System.Security.SecurityContext.Run(System.Security.SecurityContext securityContext, System.Threading.ContextCallback callback, System.Object state) in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Security/SecurityContext.cs:line 150
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state) in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Threading/ExecutionContext.cs:line 150
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef currentTaskSlot)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean bPreventDoubleExecution)
   at System.Reflection.MonoMethod.InternalInvoke(System.Object , System.Object[] , System.Exception ByRef )
   at System.Reflection.MonoMethod.Invoke(System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:line 222
   at System.Reflection.MethodBase.Invoke(System.Object obj, System.Object[] parameters) in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:line 115
   at GitHub.Unity.TaskSchedulerExcludingThread.ExecuteEntry(System.Threading.Tasks.Task task, Boolean flag) in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\ConcurrentExclusiveInterleave.cs:line 33
   at GitHub.Unity.TaskSchedulerExcludingThread.TaskExecuteWaitCallback(System.Object obj) in C:\Users\Spade\Projects\GitHub\UnityInternal\src\GitHub.Api\NewTaskSystem\ConcurrentExclusiveInterleave.cs:line 49