Fix how the git install and executable paths are set

We want the GitExecutablePath to reflect where the binary was found,
and the GitInstallPath to reflect where it is installed. When the
binary is a symlink (like /usr/local/bin/git -> ../Cellar/git/2.12.2/bin/git),
this means GitExecutablePath will have /usr/local/bin/git and
GitInstallPath will have /usr/local/Cellar/git/2.12.2/bin

src/GitHub.Api/Git/GitClient.cs

@@ -145,7 +145,7 @@ namespace GitHub.Unity
             if (path == null)
                 path = await new FindExecTask("git", taskManager.Token).StartAwait();
 
-            return path.Resolve();
+            return path;
         }
 
         public bool ValidateGitInstall(NPath path)

src/GitHub.Api/Platform/DefaultEnvironment.cs

@@ -120,7 +120,7 @@ namespace GitHub.Unity
                 if (String.IsNullOrEmpty(gitExecutablePath))
                     GitInstallPath = null;
                 else
-                    GitInstallPath = GitExecutablePath.Parent.Parent;
+                    GitInstallPath = GitExecutablePath.Resolve().Parent.Parent;
             }
         }