sync: Re-raise KeyboardInterrupt in main process

When running sync -j1, worker functions run directly in the main
process. Swallowing KeyboardInterrupt causes the loop to continue to the
next project instead of aborting.

Re-raise KeyboardInterrupt if running in the MainProcess, while
maintaining the suppression of stack traces in worker processes.

Bug: 468170157
Change-Id: I156d66bc209a265f7fa25eea0eb88737d1b51a34
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/581342
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

command.py

@@ -101,6 +101,11 @@ class Command:
     def WantPager(self, _opt):
         return False
 
+    @staticmethod
+    def is_multiprocessing_active() -> bool:
+        """Whether the current process is a worker in a pool."""
+        return multiprocessing.current_process().name != "MainProcess"
+
     def ReadEnvironmentOptions(self, opts):
         """Set options from environment variables."""