thanks fjl, but... although the original report is not incredibly clear
about this, the issue is actually that get_partial_completion_input_state
inserts the *wrong* separator, not that it should not insert a separator
at all.  for most of the callers of get_partial_completion_input_state, a
space is the appropriate separator, for example, when completing on the
name of a webjump.  but when completing on a file path, one would expect
it to insert the appropriate path component separator (\ or /) depending
on the OS.  if i recall correctly from the last time i was reading through
this part of conkeror, my determination was that we needed a completer
which was aware of the file system structure, in that it could tell the
difference between a file and a directory, insert appropriate separators,
display appropriate error messages, etc, but that to implement such a
completer without huge redundancy, the system will need a little bit of
refactoring.