git --diff: fix traceback when getting mode change
- use the manifest instead of the mode in the working dir
if the diff is against two revisions
- add a testcase
% init
% commit
adding a
% annotate -c
8435f90966e4: a
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Thu Jan 01 00:00:01 1970 +0000: a
% annotate -n
0: a
% annotate -u
nobody: a
% annotate -cdnu
nobody 0 8435f90966e4 Thu Jan 01 00:00:01 1970 +0000: a