#9 Filter for listing

[Stefan Wehr]

It should be possible to specify a filter for the
listing: show only the modified files, show only the
unknown files, show only conflicts,...

I want to set the filter per command as well as per
session:

list M -- shows only modified files
sim-update M -- simulates an update and shows only the
modified files
set-filter M -- all further operations with the
listing show only the modified files (if no other
filter is given for the operation)