vi mode completion: use APPEND when e_nlist is 0 (#493)

Attempting to tab-complete the only member of a directory while in
vi mode switches to "control" under the following conditions:

 1. pwd is outside of the directory

 2. No part of the basename has been typed in (e.g., starting with
    vim TEST/t will autocomplete to vim TEST/test, but starting
    with vim TEST/ will not).

Also, trying to type with "input" or "replace" fails afterwards
until a new line has been generated (either with ctrl + c or the
j/k keys in control mode).

src/cmd/ksh93/edit/vi.c: textmod():
- The strange completion behavior in vi mode for single-member
  directories appears to be at least partially due to completion
  failing to use \ or * mode from outside of pwd. Checking for
  e_nlist directly from vi.c — 0 unless in = mode — allows for
  entering the else loop at line 2575 to reach APPEND mode.

Resolves: https://github.com/ksh93/ksh/issues/485

NEWS

@@ -8,6 +8,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Fixed a bug introduced on 2022-02-08 where $PPID was incorrect when a script
   without a #! path was executed.
 
+- Fixed a completion bug in the vi line editor. It now correctly completes
+  the sole member of a directory when tab is pressed immediately after /.
+  Thanks to K. Eugene Carlson for the report and the fix.
+
 2022-07-26:
 
 - Fixed incorrect handling of initial zeros in test/[ and [[ arithmetic