bug#3348: 23.0.92.1; Ediff: Creating a multi-file patch

[Jon Strait]

The modified flag is not being set on the patch results buffer when 
ediff-collect-custom-diffs is rerun.

1. Mark the sessions within an ediff-directories session.
2. Run ediff-collect-custom-diffs, which creates a multi-file     patch 
and displays the buffer with this patch.
3. Save this buffer to a file 'foo.patch' with write-file.
4. Change one of the files included in the session group.
5. Run ediff-collect-custom-diffs on the same session             group 
again.
6. The new patch will be displayed as the buffer 'foo.patch',     but 
the buffer's modified flag is not set.