Remove the last uses of explicit config paths for Repository

These tests use the option to set a global configuration file with the
filemode we want. But the filemode setting should be set per-repository
as it comes down to the workdir for each repository.

Switch to setting the configuration in the local configuration, which is
also more in line with how it would exist in the wild.

Author: carlosmn

LibGit2Sharp.Tests/DiffTreeToTreeFixture.cs

@@ -1044,12 +1044,9 @@ public void ComparingReliesOnProvidedConfigEntriesIfAny()
                 repo.Config.Unset("core.filemode");
             }
 
-            SelfCleaningDirectory scd = BuildSelfCleaningDirectory();
-
-            var options = BuildFakeSystemConfigFilemodeOption(scd, true);
-
-            using (var repo = new Repository(path, options))
+            using (var repo = new Repository(path))
             {
+                SetFilemode(repo, true);
                 var changes = repo.Diff.Compare<TreeChanges>(new[] { file });
 
                 Assert.Equal(1, changes.Count());
@@ -1059,34 +1056,18 @@ public void ComparingReliesOnProvidedConfigEntriesIfAny()
                 Assert.Equal(Mode.NonExecutableFile, change.Mode);
             }
 
-            options = BuildFakeSystemConfigFilemodeOption(scd, false);
-
-            using (var repo = new Repository(path, options))
+            using (var repo = new Repository(path))
             {
+                SetFilemode(repo, false);
                 var changes = repo.Diff.Compare<TreeChanges>(new[] { file });
 
                 Assert.Equal(0, changes.Count());
             }
         }
 
-        private RepositoryOptions BuildFakeSystemConfigFilemodeOption(
-            SelfCleaningDirectory scd,
-            bool value)
+        void SetFilemode(Repository repo, bool value)
         {
-            Directory.CreateDirectory(scd.DirectoryPath);
-
-            var options = new RepositoryOptions
-                              {
-                                  SystemConfigurationLocation = Path.Combine(
-                                      scd.RootedDirectoryPath, "fake-system.config")
-                              };
-
-            StringBuilder sb = new StringBuilder()
-                .AppendFormat("[core]{0}", Environment.NewLine)
-                .AppendFormat("filemode = {1}{0}", Environment.NewLine, value);
-            Touch("", options.SystemConfigurationLocation, sb.ToString());
-
-            return options;
+            repo.Config.Set("core.filemode", value);
         }
 
         [Fact]

LibGit2Sharp.Tests/DiffWorkdirToIndexFixture.cs

@@ -127,12 +127,9 @@ public void ComparingReliesOnProvidedConfigEntriesIfAny()
                 repo.Config.Unset("core.filemode", ConfigurationLevel.Local);
             }
 
-            SelfCleaningDirectory scd = BuildSelfCleaningDirectory();
-
-            var options = BuildFakeSystemConfigFilemodeOption(scd, true);
-
-            using (var repo = new Repository(path, options))
+            using (var repo = new Repository(path))
             {
+                SetFilemode(repo, true);
                 var changes = repo.Diff.Compare<TreeChanges>(new[] { file });
 
                 Assert.Equal(1, changes.Count());
@@ -142,34 +139,18 @@ public void ComparingReliesOnProvidedConfigEntriesIfAny()
                 Assert.Equal(Mode.NonExecutableFile, change.Mode);
             }
 
-            options = BuildFakeSystemConfigFilemodeOption(scd, false);
-
-            using (var repo = new Repository(path, options))
+            using (var repo = new Repository(path))
             {
+                SetFilemode(repo, false);
                 var changes = repo.Diff.Compare<TreeChanges>(new[] { file });
 
                 Assert.Equal(0, changes.Count());
             }
         }
 
-        private RepositoryOptions BuildFakeSystemConfigFilemodeOption(
-            SelfCleaningDirectory scd,
-            bool value)
+        void SetFilemode(Repository repo, bool value)
         {
-            Directory.CreateDirectory(scd.DirectoryPath);
-
-            var options = new RepositoryOptions
-            {
-                SystemConfigurationLocation = Path.Combine(
-                    scd.RootedDirectoryPath, "fake-system.config")
-            };
-
-            StringBuilder sb = new StringBuilder()
-                .AppendFormat("[core]{0}", Environment.NewLine)
-                .AppendFormat("filemode = {1}{0}", Environment.NewLine, value);
-            File.WriteAllText(options.SystemConfigurationLocation, sb.ToString());
-
-            return options;
+            repo.Config.Set("core.filemode", value);
         }
 
         [Fact]