fix traceback of extdiff after a merge
- files that comes from a different branch are marked as modified
but aren't present in the original manifest
- add a testcase for extdiff and for regular diff
abort: impossible time zone offset: 4444444
transaction abort!
rollback completed
abort: invalid date: '1\t15.1' see hg(1) manual page for details
transaction abort!
rollback completed
abort: invalid date: 'foo bar' see hg(1) manual page for details
transaction abort!
rollback completed
abort: invalid date: ' 1 4444' see hg(1) manual page for details
transaction abort!
rollback completed
abort: date exceeds 32 bits: 111111111111
transaction abort!
rollback completed
abort: No such file or directory: .../test/bar