patch 8.0.0876: backslashes and wildcards in backticks don't work

Problem:    MS-Windows: Backslashes and wildcards in backticks don't work.
Solution:   Do not handle backslashes inside backticks in the wrong place.
            (Yasuhiro Matsumoto, closes #1942)

src/os_mswin.c

@@ -450,6 +450,15 @@ slash_adjust(char_u *p)
 {
     if (path_with_url(p))
 	return;
+
+    if (*p == '`')
+    {
+	/* don't replace backslash in backtick quoted strings */
+	int len = STRLEN(p);
+	if (len > 2 && *(p + len - 1) == '`')
+	    return;
+    }
+
     while (*p)
     {
 	if (*p == psepcN)