Fix vi mode tab completion with spaces (#413)

Attempting to complete file names in vi mode using tab completion can
fail if the last character on the command line is a space. Reproducer
(note that this bug doesn't occur in emacs mode):
   $ set -o vi
   $ mkdir '/tmp/foo bar'
   $ test -d /tmp/foo\ <Tab>

src/cmd/ksh93/edit/vi.c:
- Don't disable tab completion or reset the tab count just because the
  last character on the command line is a space. This bugfix was
  backported from ksh93v- 2014-06-06.

src/cmd/ksh93/tests/pty.sh:
- Add a regression test for the tab completion bug.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-01-04:
+
+- Fixed an issue in vi mode that caused tab completion to fail if the
+  last character on the command line was a space.
+
 2021-12-29:
 
 - Fixed numerous build errors that prevented ksh from running on Haiku OS.