sync: Improve UI and error reporting for interleaved mode

This fixes two issues:
1. the progress bar could show a count greater than the total if new projects were discovered mid-sync. Update the progress bar total dynamically
2. Make "Stall detected" error message more actionable

Bug: 432206932
Change-Id: Ie2a4ada5b1770cae0302fb06590641c522cbb7e7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/491941
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Scott Lee <ddoman@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

progress.py

@@ -119,6 +119,11 @@ class Progress:
         if not quiet and show_elapsed:
             self._update_thread.start()
 
+    def update_total(self, new_total):
+        """Updates the total if the new total is larger."""
+        if new_total > self._total:
+            self._total = new_total
+
     def _update_loop(self):
         while True:
             self.update(inc=0)