Handle files with a backtick in the name

GitLink uses the `Uri` APIs in order to calculate relative file paths.
This API will escape a number of characters, including backtick, when
converting from `string` to `Uri`. These characters are not unescaped
when converting from `Uri` to `string`. Hence the paths produced from
GetRelativePath are incorrect and GitLink can't process the files.

Fixed to unescape the characters once the relative path is calculated.

closes #163

src/GitLink/Extensions/RepositoryExtensions.cs

@@ -74,7 +74,7 @@ private static string GetRelativePath(string target, string relativeTo)
 
             Uri baseUri = new Uri(relativeTo, UriKind.Absolute);
             Uri targetUri = new Uri(target, UriKind.Absolute);
-            return baseUri.MakeRelativeUri(targetUri).ToString();
+            return Uri.UnescapeDataString(baseUri.MakeRelativeUri(targetUri).ToString());
         }
     }
 }