Make sure that GitRepositoryProvider.repositoryForDirectorySync() returns null rather than throws.

The UI locks up if this method does not return.

spec/git-repository-provider-spec.coffee

@@ -54,3 +54,33 @@ describe "GitRepositoryProvider", ->
           directory = new Directory dirPath
           provider.repositoryForDirectory(directory).then (result) ->
             expect(result).toBe null
+
+    describe "when specified a Directory that throws an exception when explored", ->
+      it "catches the exception and returns null for the sync implementation", ->
+        provider = new GitRepositoryProvider atom.project
+
+        # Tolerate an implementation of Directory whose sync methods are unsupported.
+        subdirectory = existsSync: ->
+        spyOn(subdirectory, "existsSync").andThrow("sync method not supported")
+        directory = getSubdirectory: ->
+        spyOn(directory, "getSubdirectory").andReturn(subdirectory)
+
+        provider = new GitRepositoryProvider atom.project
+        repo = provider.repositoryForDirectorySync(directory)
+        expect(repo).toBe null
+        expect(directory.getSubdirectory).toHaveBeenCalledWith(".git")
+
+      it "catches the exception and returns a Promise that resolves to null for the async implementation", ->
+        provider = new GitRepositoryProvider atom.project
+
+        # Tolerate an implementation of Directory whose sync methods are unsupported.
+        subdirectory = existsSync: ->
+        spyOn(subdirectory, "existsSync").andThrow("sync method not supported")
+        directory = getSubdirectory: ->
+        spyOn(directory, "getSubdirectory").andReturn(subdirectory)
+
+        waitsForPromise ->
+          provider = new GitRepositoryProvider atom.project
+          provider.repositoryForDirectory(directory).then (repo) ->
+            expect(repo).toBe null
+            expect(directory.getSubdirectory).toHaveBeenCalledWith(".git")

src/git-repository-provider.coffee

@@ -51,6 +51,14 @@ class GitRepositoryProvider
   # * {GitRepository} if the given directory has a Git repository.
   # * `null` if the given directory does not have a Git repository.
   repositoryForDirectorySync: (directory) ->
+    # Ensure that this method does not throw.
+    try
+      @repositoryForDirectorySyncInternal(directory)
+    catch e
+      # TODO: Log error.
+      null
+
+  repositoryForDirectorySyncInternal: (directory) ->
     # Only one GitRepository should be created for each .git folder. Therefore,
     # we must check directory and its parent directories to find the nearest
     # .git folder.