No completion before 1st char or in empty line (re: b6c8bb7b) (#523)

There are still two contexts in which the unhelpful "alias +
builtin + path" completion occurs:

* After the start of an empty line
* After the start of a line, but before the first character

To stop completion from being attempted in these contexts, a loop
is added to vi.c and emacs.c that looks over the previous positions
(current position inclusive) and allows the completion process to
continue once it finds the first non-space character. The existing
placement restrictions remain in case the cursor arrives in a
negative position via a bug.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/cmd/ksh93/edit/emacs.c

@@ -1001,8 +1001,19 @@ static int escape(register Emacs_t* ep,register genchar *out,int count)
 			/* FALLTHROUGH */
 		case '*':		/* filename expansion */
 		case '=':	/* escape = - list all matching file names */
+		{
+			char allempty = 1;
+			int x;
 			ep->mark = cur;
-			if(cur<1)
+			for(x=0; x < cur; x++)
+			{
+				if(!isspace(out[x]))
+				{
+					allempty = 0;
+					break;
+				}
+			}
+			if(cur<1 || allempty)
 			{
 				beep();
 				return(-1);
@@ -1040,6 +1051,7 @@ static int escape(register Emacs_t* ep,register genchar *out,int count)
 				draw(ep,UPDATE);
 			}
 			return(-1);
+		}
 
 		/* search back for character */
 		case cntl(']'):	/* feature not in book */

src/cmd/ksh93/edit/vi.c

@@ -1537,6 +1537,22 @@ static void getline(register Vi_t* vp,register int mode)
 			return;
 
 		case '\t':		/** command completion **/
+		{
+			char allempty = 1;
+			int x;
+			for(x=0; x <= cur_virt; x++)
+			{
+				if(!isspace(virtual[x]))
+				{
+					allempty = 0;
+					break;
+				}
+			}
+			if(allempty)
+			{
+				ed_ringbell();
+				break;
+			}
 			if(sh_isoption(SH_VI) &&
 				mode != SEARCH &&
 				last_virt >= 0 &&
@@ -1566,6 +1582,7 @@ static void getline(register Vi_t* vp,register int mode)
 				break;
 			}
 			/* FALLTHROUGH */
+		}
 		default:
 		fallback:
 			if( mode == REPLACE )
@@ -2545,7 +2562,18 @@ addin:
 	case '*':		/** do file name expansion in place **/
 	case '\\':		/** do file name completion in place **/
 	case '=':		/** list file name expansions **/
-		if( cur_virt == INVALID )
+	{
+		char allempty = 1;
+		int x;
+		for(x=0; x <= cur_virt; x++)
+		{
+			if(!isspace(virtual[x]))
+			{
+				allempty = 0;
+				break;
+			}
+		}
+		if(cur_virt == INVALID || allempty)
 			return(BAD);
 		/* FALLTHROUGH */
 		save_v(vp);
@@ -2584,6 +2612,7 @@ addin:
 			return(APPEND);
 		}
 		break;
+	}
 
 	case '@':		/** macro expansion **/
 		if( mode )

src/cmd/ksh93/sh.1

@@ -4987,6 +4987,8 @@ will provide a numbered list of matching alternatives.
 A specific selection can be made by entering the
 selection number followed by a
 .IR tab .
+Neither completion nor listing operations are attempted before
+the first character in a line.
 .SS Key Bindings.
 The
 .B
@@ -5581,10 +5583,9 @@ or
 character.
 .TP 10
 .BI ^I  " tab"
-Except at the start of the line, attempts command or
+Attempts command or file name completion as described
-file name completion as described above and returns to
+above and returns to input mode. If a partial completion
-input mode. If a partial completion occurs, repeating
+occurs, repeating this will behave as if
-this will behave as if
 .B =
 were entered from control mode.
 If no match is found or entered after