• Error in external git client (for example SourceTree, GitKraken):

  Rename from 'C:/path/to/repository/.git/index.lock' to 'C:/path/to/repository/.git/index' failed. 
  fatal: unable to write new index file
  

• OS: Windows
• Project opened in Visual Studio Code
• Remote connection type: WSL
• Visual Studio Code Extension: Remote - WSL

Here is the fix:

F1>”Open Remote Settings (WSL: Ubuntu) and add the following lines to your settings.json.

"remote.WSL.fileWatcher.polling": true,
"files.watcherExclude": {
    "**/.git/**": true
}