filterfiles: Search as long as the target is a prefix of current. filterfiles was failing to find files for directory arguments if another file existed that started with the directory name and sorted earlier. For example, a manifest of ('foo.h', 'foo/foo') would cause filterfiles('foo') to return nothing. This resolves issue #294.

adding a
adding b
adding t/x
a
NONEXISTENT: No such file or directory
a
b
t/x
a: No such file or directory
NONEXISTENT: No such file or directory
b
t/x
a
NONEXISTENT: No such file in rev 9449ae447738
a
t/x
b
% -I/-X with relative path should work
b
t/x
t/x