Add symlink resolver so we can figure out paths properly

Fixes #192

On non-windows, git and other files might be symlinks, so when
we're using them as a base for determining parent directories,
we may need to resolve the symlink first. The rules for automatically
doing symlink resolution are tricky, so for now just do it manually.

src/GitHub.Api/Git/GitClient.cs

@@ -134,13 +134,18 @@ namespace GitHub.Unity
 
         private async Task<NPath> LookForSystemGit()
         {
-            if (environment.IsMac)
+            NPath path = null;
+            if (!environment.IsWindows)
             {
-                var path = "/usr/local/bin/git".ToNPath();
-                if (path.FileExists())
-                    return path;
+                var p = new NPath("/usr/local/bin/git");
+                if (p.FileExists())
+                    path = p;
             }
-            return await new FindExecTask("git", taskManager.Token).StartAwait();
+
+            if (path == null)
+                path = await new FindExecTask("git", taskManager.Token).StartAwait();
+
+            return path.Resolve();
         }
 
         public bool ValidateGitInstall(NPath path)

src/GitHub.Api/GitHub.Api.csproj

@@ -69,6 +69,9 @@
     <Reference Include="Mono.Security">
       <HintPath>$(SolutionDir)lib\Mono.Security.dll</HintPath>
     </Reference>
+    <Reference Include="Mono.Posix">
+      <HintPath>$(SolutionDir)lib\Mono.Posix.dll</HintPath>
+    </Reference>
     <Reference Include="Rackspace.Threading, Version=2.0.0.0, Culture=neutral, PublicKeyToken=bb62785d398726f0, processorArchitecture=MSIL">
       <HintPath>$(SolutionDir)\packages\TunnelVisionLabs.Threading.2.0.0-unity\lib\net35-client\Rackspace.Threading.dll</HintPath>
       <Private>True</Private>

src/GitHub.Api/IO/NiceIO.cs

@@ -283,6 +283,8 @@ GitHub.Unity
 
         public bool DirectoryExists(NPath append)
         {
+            if (append == null)
+                return FileSystem.DirectoryExists(ToString());
             return FileSystem.DirectoryExists(Combine(append).ToString());
         }
 
@@ -295,6 +297,8 @@ GitHub.Unity
 
         public bool FileExists(NPath append)
         {
+            if (append == null)
+                return FileSystem.FileExists(ToString());
             return FileSystem.FileExists(Combine(append).ToString());
         }
 
@@ -1018,6 +1022,14 @@ GitHub.Unity
                 return null;
             return new NPath(path);
         }
+
+        public static NPath Resolve(this NPath path)
+        {
+            if (path == null || DefaultEnvironment.OnWindows || path.IsRelative || !path.FileExists())
+                return path;
+
+            return new NPath(Mono.Unix.UnixPath.GetCompleteRealPath(path.ToString()));
+        }
     }
 
     public enum SlashMode

src/GitHub.Api/Platform/DefaultEnvironment.cs

@@ -117,35 +117,14 @@ namespace GitHub.Unity
             set
             {
                 gitExecutablePath = value;
-                gitInstallPath = null;
+                if (String.IsNullOrEmpty(gitExecutablePath))
+                    GitInstallPath = null;
+                else
+                    GitInstallPath = GitExecutablePath.Parent.Parent;
             }
         }
 
-        private NPath gitInstallPath;
-        public NPath GitInstallPath
-        {
-            get
-            {
-                if (gitInstallPath == null)
-                {
-
-                    if (!String.IsNullOrEmpty(GitExecutablePath))
-                    {
-                        if (IsWindows)
-                        {
-                            gitInstallPath = GitExecutablePath.Parent.Parent;
-                        }
-                        else
-                        {
-                            gitInstallPath = GitExecutablePath.Parent;
-                        }
-                    }
-                    else
-                        gitInstallPath = GitExecutablePath;
-                }
-                return gitInstallPath;
-            }
-        }
+        public NPath GitInstallPath { get; private set; }
 
         public NPath RepositoryPath { get; private set; }
         public IRepository Repository { get; set; }