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
changeset: 0:9c2cf2b35aa7
user: test
date: Mon Jan 12 13:46:40 1970 +0000
summary: add foo
changeset: 1:3756a9556b89
tag: tip
user: test
date: Mon Jan 12 13:46:41 1970 +0000
summary: change foo
2 revisions:
9c2cf2b35aa7
3756a9556b89