feat(explorer): add filesystem watchers (#1304)

* feat(explorer): add experimental watchers

This commit introduces watchers to update the tree.
This behavior is introduced behind an "filesystem_watchers" option
which should prevent instabilities.
It will become the default at some point.

Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/git/runner.lua

@@ -53,7 +53,9 @@ end
 
 function Runner:_log_raw_output(output)
   if output and type(output) == "string" then
-    log.raw("git", "%s", output)
+    -- TODO put this back after watcher feature completed
+    -- log.raw("git", "%s", output)
+    log.line("git", "done")
   end
 end