emacs: Fix digits input after completion (re: 16e4824c, e8b3274a)

Immediately after tab-completing the name of a directory, it is
not possible to type digits after the slash; ksh eats them as it
parses them as a menu selection for a nonexistent menu.

Reproducer:
$ mkdir -p emacstest/123abc
$ cd emacste[tab]123abc

Actual results:
$ cd emacstest/abc

Expected results:
$ cd emacstest/123abc

Workarounds are to press a non-numeric key followed by backspace,
or hit [tab] again to get a list of options.

Originally reported by Arnon Weinberg, 2012-12-23 07:15:19 UTC, at:
https://bugzilla.redhat.com/889745

The fix had been partially backported from ksh 93v- by AT&T
(16e4824c), which made things worse, so it was reverted (e8b3274a).
This commit backports a slightly edited version of the complete
fix. Thanks to @JohnoKing for finding the correct code. Discussion:
https://github.com/ksh93/ksh/issues/198#issuecomment-820178514

src/cmd/ksh93/edit/emacs.c: escape():
- Backport the fix for this bug that was implemented in ksh 93v-
  alpha 2013-10-10. Immediately after a slash, do not stay in "\"
  mode (file name completion) and reset the tab count.

src/cmd/ksh93/tests/pty.sh:
- Test the fix.

Resolves: https://github.com/ksh93/ksh/issues/198

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.
 
+2021-04-16:
+
+- Fixed a bug in emacs mode: after using tab completion to complete the name
+  of a directory, it was not possible to type numbers after the slash.
+
 2021-04-15:
 
 - Fixed an optimization bug that caused the <>; redirection operator to fail