Fix crash on diff of new empty file

When adding (or presumably removing) an empty file, there is no
diff header, but that's where Gertty was getting the filenames.
Instead, get them from the git blob itself.

Change-Id: Ib2908aaf1d7554df53b5fb262017d85f3a560066
Story: 161

gertty/gitrepo.py

@@ -45,6 +45,11 @@ class GitTimeZone(datetime.tzinfo):
         return None
 
 
+class CommitBlob(object):
+    def __init__(self):
+        self.path = '/COMMIT_MSG'
+
+
 class CommitContext(object):
     """A git.diff.Diff for commit messages."""
 
@@ -92,6 +97,13 @@ class CommitContext(object):
         :param new: A git.objects.commit object.
         """
         self.rename_from = self.rename_to = None
+        if old is None:
+            self.new_file = True
+        else:
+            self.new_file = False
+        self.deleted_file = False
+        self.a_blob = CommitBlob()
+        self.b_blob = CommitBlob()
         self.diff = ''.join(difflib.unified_diff(
             self.decorateMessage(old), self.decorateMessage(new),
             fromfile="/a/COMMIT_MSG", tofile="/b/COMMIT_MSG"))
@@ -349,6 +361,14 @@ class Repo(object):
         for diff_context in contexts:
             # Each iteration of this is a file
             f = DiffFile()
+            if diff_context.a_blob:
+                f.oldname = diff_context.a_blob.path
+            if diff_context.b_blob:
+                f.newname = diff_context.b_blob.path
+            if diff_context.new_file:
+                f.oldname = 'Empty file'
+            if diff_context.deleted_file:
+                f.newname = 'Empty file'
             files.append(f)
             if diff_context.rename_from:
                 f.oldname = diff_context.rename_from
@@ -361,14 +381,8 @@ class Repo(object):
             for i, line in enumerate(diff_lines):
                 last_line = (i == len(diff_lines)-1)
                 if line.startswith('---'):
-                    f.oldname = line[6:]
-                    if line[4:] == '/dev/null':
-                        f.oldname = 'Empty file'
                     continue
                 if line.startswith('+++'):
-                    f.newname = line[6:]
-                    if line[4:] == '/dev/null':
-                        f.newname = 'Empty file'
                     continue
                 if line.startswith('@@'):
                     #socket.sendall(line)