hg2git.py: Add simple delta revision feed
Now we have three methods of feeding out changes
1) full for first revision or
2) thorough delta for merges (compare checksums with all parents) or
3) simple delta else (only got with manifest)
This requires some cleanup so that we have only place where we actually
call the appropriate dumping method.
The export_file_contents() method now also sorts its file list before
writing out anything as this seems to speed up hg data retrival a bit.
Signed-off-by: Rocco Rutte <pdmef@gmx.net>