patch 9.1.0535: newline escape wrong in ex mode

Problem:  newline escape wrong in ex mode (Konrad Schwarz)
Solution: partly revert patch 7.3.014, remove backslash in front of a
          newline when not in prompt mode in ex line mode
          (Mohamed Akram)

This fixes newline escaping to allow passing multiple commands to
":global", multiple lines to shell commands, and ending lines in append
mode with backslashes. This should fix a POSIX/(traditional) VI
incompatiblity.

This reverts a previous incorrect attempt at patch v7.3.014 to fix
append mode which removed half of trailing backslashes which lead to,
eg. the following two commands being parsed as having a different number
of backslashes:

```
!echo foo\\\
```

```
!echo foo\\ \
```

fixes: #6135
fixes: #7244
closes: #15120

Signed-off-by: Mohamed Akram <mohd.akram@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/ex_getln.c b/src/ex_getln.c
index f05259b..51a38e5 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -3138,31 +3138,15 @@
 	windgoto(msg_row, msg_col);
 	pend = (char_u *)(line_ga.ga_data) + line_ga.ga_len;
 
-	// We are done when a NL is entered, but not when it comes after an
-	// odd number of backslashes, that results in a NUL.
-	if (line_ga.ga_len > 0 && pend[-1] == '\n')
+	// We are done when a NL is entered, but not when it comes after a
+	// backslash in prompt mode.
+	if (line_ga.ga_len > 0 && pend[-1] == '\n'
+		&& (line_ga.ga_len <= 1 || pend[-2] != '\\' || !promptc))
 	{
-	    int bcount = 0;
-
-	    while (line_ga.ga_len - 2 >= bcount && pend[-2 - bcount] == '\\')
-		++bcount;
-
-	    if (bcount > 0)
-	    {
-		// Halve the number of backslashes: "\NL" -> "NUL", "\\NL" ->
-		// "\NL", etc.
-		line_ga.ga_len -= (bcount + 1) / 2;
-		pend -= (bcount + 1) / 2;
-		pend[-1] = '\n';
-	    }
-
-	    if ((bcount & 1) == 0)
-	    {
-		--line_ga.ga_len;
-		--pend;
-		*pend = NUL;
-		break;
-	    }
+	    --line_ga.ga_len;
+	    --pend;
+	    *pend = NUL;
+	    break;
 	}
     }