fix(completion): don't add backslashes to runtime pattern (#24296)

Problem:    Bashslashes added as regexp in runtime completion may be
            treated as path separator with some 'isfname' value.
Solution:   Make curly braces work for runtime completion and use it.

src/nvim/path.c

@@ -1163,7 +1163,7 @@ static bool has_env_var(char *p)
 
 // Return true if "p" contains a special wildcard character, one that Vim
 // cannot expand, requires using a shell.
-static bool has_special_wildchar(char *p)
+static bool has_special_wildchar(char *p, int flags)
 {
   for (; *p; MB_PTR_ADV(p)) {
     // Disallow line break characters.
@@ -1174,6 +1174,10 @@ static bool has_special_wildchar(char *p)
     if (*p == '\\' && p[1] != NUL && p[1] != '\r' && p[1] != '\n') {
       p++;
     } else if (vim_strchr(SPECIAL_WILDCHAR, (uint8_t)(*p)) != NULL) {
+      // Need a shell for curly braces only when including non-existing files.
+      if (*p == '{' && !(flags & EW_NOTFOUND)) {
+        continue;
+      }
       // A { must be followed by a matching }.
       if (*p == '{' && vim_strchr(p, '}') == NULL) {
         continue;
@@ -1233,7 +1237,7 @@ int gen_expand_wildcards(int num_pat, char **pat, int *num_file, char ***file, i
   // avoids starting the shell for each argument separately.
   // For `=expr` do use the internal function.
   for (int i = 0; i < num_pat; i++) {
-    if (has_special_wildchar(pat[i])
+    if (has_special_wildchar(pat[i], flags)
         && !(vim_backtick(pat[i]) && pat[i][1] == '=')) {
       return os_expand_wildcards(num_pat, pat, num_file, file, flags);
     }

src/nvim/runtime.c

@@ -338,7 +338,7 @@ int do_in_path(char *path, char *name, int flags, DoInRuntimepathCB callback, vo
           }
 
           int ew_flags = ((flags & DIP_DIR) ? EW_DIR : EW_FILE)
-                         | (flags & DIP_DIRFILE) ? (EW_DIR|EW_FILE) : 0;
+                         | ((flags & DIP_DIRFILE) ? (EW_DIR|EW_FILE) : 0);
 
           // Expand wildcards, invoke the callback for each match.
           if (gen_expand_wildcards(1, &buf, &num_files, &files, ew_flags) == OK) {
@@ -1222,9 +1222,9 @@ static void ExpandRTDir_int(char *pat, size_t pat_len, int flags, bool keep_ext,
     bool expand_dirs = false;
 
     if (*dirnames[i] == NUL) {  // empty dir used for :runtime
-      snprintf(tail, tail_buflen, "%s*.\\(vim\\|lua\\)", pat);
+      snprintf(tail, tail_buflen, "%s*.{vim,lua}", pat);
     } else {
-      snprintf(tail, tail_buflen, "%s/%s*.\\(vim\\|lua\\)", dirnames[i], pat);
+      snprintf(tail, tail_buflen, "%s/%s*.{vim,lua}", dirnames[i], pat);
     }
 
 expand:

test/functional/lua/runtime_spec.lua

@@ -18,7 +18,10 @@ describe('runtime:', function()
     io.open(init, 'w'):close()  --  touch init file
     clear{args = {'-u', init}}
     exec('set rtp+=' .. plug_dir)
-    exec('set completeslash=slash')
+    exec([[
+      set completeslash=slash
+      set isfname+=(,)
+    ]])
   end)
 
   teardown(function()