vim-patch:8.2.3385: escaping for fish shell does not work properly

Problem:    Escaping for fish shell does not work properly.
Solution:   Insert a backslash before a backslash. (Jason Cox, closes vim/vim#8810)
6e82351130

src/nvim/strings.c

@@ -190,6 +190,7 @@ char_u *vim_strsave_shellescape(const char_u *string,
   char_u      *escaped_string;
   size_t l;
   int csh_like;
+  bool fish_like;
 
   /* Only csh and similar shells expand '!' within single quotes.  For sh and
    * the like we must not put a backslash before it, it will be taken
@@ -197,6 +198,10 @@ char_u *vim_strsave_shellescape(const char_u *string,
    * Csh also needs to have "\n" escaped twice when do_special is set. */
   csh_like = csh_like_shell();
 
+  // Fish shell uses '\' as an escape character within single quotes, so '\'
+  // itself must be escaped to get a literal '\'.
+  fish_like = fish_like_shell();
+
   /* First count the number of extra bytes required. */
   size_t length = STRLEN(string) + 3;       // two quotes and a trailing NUL
   for (const char_u *p = string; *p != NUL; MB_PTR_ADV(p)) {
@@ -220,6 +225,9 @@ char_u *vim_strsave_shellescape(const char_u *string,
       ++length;                         /* insert backslash */
       p += l - 1;
     }
+    if (*p == '\\' && fish_like) {
+      length++;  // insert backslash
+    }
   }
 
   /* Allocate memory for the result and fill it. */
@@ -267,6 +275,10 @@ char_u *vim_strsave_shellescape(const char_u *string,
         *d++ = *p++;
       continue;
     }
+    if (*p == '\\' && fish_like) {
+      *d++ = '\\';
+      *d++ = *p++;
+    }
 
     MB_COPY_CHAR(p, d);
   }