fix(fs): duplicate path separator #21509

Fixes #21497

runtime/lua/vim/fs.lua

@@ -74,7 +74,7 @@ end
 
 ---@private
 local function join_paths(...)
-  return table.concat({ ... }, '/')
+  return (table.concat({ ... }, '/'):gsub('//+', '/'))
 end
 
 --- Return an iterator over the files and directories located in {path}

test/functional/lua/fs_spec.lua

@@ -230,6 +230,11 @@ describe('vim.fs', function()
         local dir, nvim = ...
         return vim.fs.find(nvim, { path = dir, type = 'file' })
       ]], test_build_dir, nvim_prog_basename))
+      eq({nvim_dir}, exec_lua([[
+        local dir = ...
+        local parent, name = dir:match('^(.*/)([^/]+)$')
+        return vim.fs.find(name, { path = parent, upward = true, type = 'directory' })
+      ]], nvim_dir))
     end)
 
     it('accepts predicate as names', function()