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/Commands/QueryCommitFullMessage.cs

@@ -0,0 +1,19 @@
+namespace SourceGit.Commands
+{
+    public class QueryCommitFullMessage : Command
+    {
+        public QueryCommitFullMessage(string repo, string sha)
+        {
+            WorkingDirectory = repo;
+            Context = repo;
+            Args = $"show --no-show-signature --pretty=format:%B -s {sha}";
+        }
+
+        public string Result()
+        {
+            var rs = ReadToEnd();
+            if (rs.IsSuccess) return rs.StdOut.TrimEnd();
+            return string.Empty;
+        }
+    }
+}