Fix files from other changelists shown under "No Changelist" when filtering

[BHFock]

Problem

When filtering git cl st by changelist name with --include-no-cl, files belonging to other (filtered-out) changelists were incorrectly listed under "No Changelist".

For example, with leak-a.txt in list1 and leak-b.txt in list2:

git cl st list1 --include-no-cl

showed leak-b.txt under "No Changelist", even though it is assigned to list2.

Cause

clutil_show_active_changelists only recorded a file as assigned when its changelist was actually displayed. The bookkeeping that populates assigned_files sat after the selected_names filter, so files in changelists that weren't shown never got recorded — and the "No Changelist" section, which lists everything not in assigned_files, then treated them as unassigned.

The bug was not specific to untracked files: a tracked modification ([ M]) in a filtered-out changelist leaked the same way. Untracked files just made it more noticeable.

Fix

Move the assigned_files bookkeeping ahead of the display filter, so all assignments are recorded regardless of which changelists are shown. Display remains filtered; only the accounting is now complete.

Test

Adds a regression test in test_basic_add_status.py that sets up two changelists, filters by one with --include-no-cl, and asserts that:

• the filtered changelist and its file are shown,
• a genuinely unassigned file does appear under "No Changelist",
• files from the filtered-out changelist (both a tracked [ M] and an untracked [??]) do not leak in.

Verified failing against the previous code and passing with the fix.