Fix diff crash on perm-only changes

The update to GitPython that supports easily detecting the filenames
in file-permissions-only changes merged a year ago.  Start using it
and raise the mininum GitPython version to one which supports it.

Change-Id: I9a5104a4b399dee0d9e029fd0b10e37b8aafd3f8

gertty/gitrepo.py

@@ -392,14 +392,8 @@ 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
-            # TODO(jeblair): if/when https://github.com/gitpython-developers/GitPython/pull/266 merges,
-            # remove above 4 lines and replace with these two:
-            # f.oldname = diff_context.a_path
-            # f.newname = diff_context.b_path
+            f.oldname = diff_context.a_path
+            f.newname = diff_context.b_path
             if diff_context.new_file:
                 f.oldname = 'Empty file'
                 f.old_empty = True

requirements.txt

@@ -2,7 +2,7 @@ pbr>=0.11,<2.0
 
 urwid>=1.2.1,!=1.3.0
 SQLAlchemy>=1.0.4
-GitPython>=0.3.2.RC1,<1.0.2
+GitPython>=0.3.7,<1.0.2
 python-dateutil
 requests>=2.5.3,<3.0.0
 ordereddict