Basic reflog support

For now it simply includes reading in the reflog as a list of
entries. This does mean that the drop/append functions don't need to be
wrapped, as we can use erlang's list handling.

It however also means that we cannot write this back out, as there is
currently no way to create an empty reflog that we can fill with our own
list.

test/repo_test.erl

@@ -59,11 +59,13 @@ ref_test(Repo) ->
     {ok, Ref1} = geef_ref:lookup(Repo, "refs/heads/other"),
     {ok, Ref2} = geef_ref:resolve(Ref0),
     {ok, Dwimed} = geef_ref:dwim(Repo, "branch"),
+    {ok, Reflog} = geef_reflog:read(Repo, "refs/heads/branch"),
     [?_assertEqual(Ref0#geef_reference.target, Id),
      ?_assertEqual(Ref1#geef_reference.target, <<"refs/heads/branch">>),
      ?_assertEqual(<<"branch">>, geef_ref:shorthand(Ref0)),
      ?_assertEqual(Ref2#geef_reference.target, Id),
      ?_assertEqual({ok, true}, geef_ref:has_log(Ref0)),
+     ?_assertEqual(1, length(Reflog)),
      ?_assertEqual(Ref0, Dwimed)].
 
 ref_iter_test(Repo) ->