enhance: only store subject in commits.

It has several advantages:

* reduce the memory costed by histories
* higher performance while parsing commits
* no need to calculate subject every time, which is invoked most frequently to render histories

src/ViewModels/Squash.cs

@@ -27,7 +27,8 @@ namespace SourceGit.ViewModels
         public Squash(Repository repo, Models.Commit head, Models.Commit parent)
         {
             _repo = repo;
-            _message = parent.Body;
+            _message = new Commands.QueryCommitFullMessage(_repo.FullPath, parent.SHA).Result();
+            
             Head = head;
             Parent = parent;
             View = new Views.Squash() { DataContext = this };