patman: Allow showing a Commit as a string

Use the subject of the Commit object when printing it out.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/commit.py b/tools/patman/commit.py
index e49bf87..5bf2b94 100644
--- a/tools/patman/commit.py
+++ b/tools/patman/commit.py
@@ -41,6 +41,9 @@
         self.rtags = collections.defaultdict(set)
         self.warn = []
 
+    def __str__(self):
+        return self.subject
+
     def AddChange(self, version, info):
         """Add a new change line to the change list for a version.