Detect local commits in PR branch.

And invite user to check out PR to a new branch. The actual checkout is
not yet implemented.

Author: grokys

src/GitHub.App/ViewModels/PullRequestDetailViewModel.cs

@@ -335,9 +335,9 @@ public async Task Load(PullRequest pullRequest, IList<PullRequestFile> files)
             {
                 var divergence = await pullRequestsService.CalculateHistoryDivergence(repository, Number);
 
-                if (divergence.BehindBy == null)
+                if (divergence.BehindBy == null || divergence.AheadBy > 0)
                 {
-                    CheckoutMode = CheckoutMode.Fetch;
+                    CheckoutMode = CheckoutMode.InvalidState;
                 }
                 else if (divergence.BehindBy == 0)
                 {
@@ -413,6 +413,7 @@ static string GetCheckoutModeDescription(CheckoutMode checkoutMode)
                 case CheckoutMode.Switch:
                     return "switch branches";
                 case CheckoutMode.Fetch:
+                case CheckoutMode.InvalidState:
                     return "checkout pull request";
                 default:
                     Debug.Fail("Invalid CheckoutMode in GetCheckoutModeDescription");

src/GitHub.Exports.Reactive/ViewModels/IPullRequestDetailViewModel.cs

@@ -61,6 +61,12 @@ public enum CheckoutMode
         /// No local branch exists for the pull request.
         /// </summary>
         Fetch,
+
+        /// <summary>
+        /// The checked out branch is marked as the branch for the pull request, but there are
+        /// local commits or the branch shares no history.
+        /// </summary>
+        InvalidState,
     }
 
     /// <summary>

src/GitHub.VisualStudio/UI/Views/PullRequestDetailView.xaml

@@ -187,7 +187,7 @@
                     </TextBlock.Style>
                 </TextBlock>
 
-                <!-- Branch checked out and neds pull -->
+                <!-- Branch checked out and needs pull -->
                 <TextBlock TextWrapping="Wrap">
                     <ui:OcticonImage Icon="sync" Foreground="Orange" Margin="0 0 0 -4"/>
                     <Run>Your local branch is behind by</Run>
@@ -221,6 +221,23 @@
                     </TextBlock.Style>
                 </TextBlock>
 
+                <!-- Local branch is checked out but is in an invalid state -->
+                <TextBlock TextWrapping="Wrap">
+                    <ui:OcticonImage Icon="alert" Foreground="Orange" Margin="0 0 0 -4"/>
+                    <Run BaselineAlignment="Top">The pull request branch has local changes.</Run>
+                    <Hyperlink Command="{Binding Checkout}">Checkout to a new branch</Hyperlink>
+                    <TextBlock.Style>
+                        <Style TargetType="TextBlock" BasedOn="{StaticResource CheckoutMessage}">
+                            <Setter Property="Visibility" Value="Collapsed"/>
+                            <Style.Triggers>
+                                <DataTrigger Binding="{Binding CheckoutMode}" Value="InvalidState">
+                                    <Setter Property="Visibility" Value="Visible"/>
+                                </DataTrigger>
+                            </Style.Triggers>
+                        </Style>
+                    </TextBlock.Style>
+                </TextBlock>
+
                 <!-- Checkout disabled message -->
                 <TextBlock Margin="0 4" TextWrapping="Wrap">
                     <ui:OcticonImage Icon="alert" Foreground="Orange" Margin="0 0 0 -4"/>

src/UnitTests/GitHub.App/ViewModels/PullRequestDetailViewModelTests.cs

@@ -153,10 +153,41 @@ public async Task CheckoutDisabledMessageShouldBeSetWhenNeedsFetchAndWorkingDire
             Assert.False(target.Checkout.CanExecute(null));
         }
 
+        [Fact]
+        public async Task CheckoutModeShouldBeInvalidStateWhenHasLocalCommits()
+        {
+            var target = CreateTarget(
+                currentBranch: "pr/123",
+                existingPrBranch: "pr/123",
+                aheadBy: 1,
+                behindBy: 2);
+            await target.Load(CreatePullRequest(), new PullRequestFile[0]);
+
+            Assert.Equal(CheckoutMode.InvalidState, target.CheckoutMode);
+            Assert.True(target.Checkout.CanExecute(null));
+        }
+
+        [Fact]
+        public async Task CheckoutDisabledMessageShouldBeSetWhenInvalidStateAndWorkingDirectoryDirty()
+        {
+            var target = CreateTarget(
+                currentBranch: "pr/123",
+                existingPrBranch: "pr/123",
+                aheadBy: 1,
+                behindBy: 2,
+                dirty: true);
+            await target.Load(CreatePullRequest(), new PullRequestFile[0]);
+
+            Assert.Equal(CheckoutMode.InvalidState, target.CheckoutMode);
+            Assert.Equal("Cannot checkout pull request as your working directory has uncommitted changes.", target.CheckoutDisabledMessage);
+            Assert.False(target.Checkout.CanExecute(null));
+        }
+
         PullRequestDetailViewModel CreateTarget(
             string currentBranch = "master",
             string existingPrBranch = null,
             bool dirty = false,
+            int aheadBy = 0,
             int behindBy = 0)
         {
             var repository = Substitute.For<ILocalRepositoryModel>();
@@ -181,6 +212,7 @@ PullRequestDetailViewModel CreateTarget(
             pullRequestService.CleanForCheckout(repository).Returns(Observable.Return(!dirty));
 
             var divergence = Substitute.For<HistoryDivergence>();
+            divergence.AheadBy.Returns(aheadBy);
             divergence.BehindBy.Returns(behindBy);
             pullRequestService.CalculateHistoryDivergence(repository, Arg.Any<int>())
                 .Returns(Observable.Return(divergence));