sync: Implement --interleaved sync worker

For each assigned project, the worker sequentially calls
Sync_NetworkHalf and Sync_LocalHalf, respecting --local-only and
--network-only flags. To prevent scrambled progress bars, all stderr
output from the checkout phase is captured (shown with --verbose).
Result objects now carry status and timing information from the worker
for state updates.

Bug: 421935613
Change-Id: I398602e08a375e974a8914e5fa48ffae673dda9b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/483301
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Scott Lee <ddoman@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

progress.py

@@ -195,6 +195,21 @@ class Progress:
                 )
             )
 
+    def display_message(self, msg):
+        """Clears the current progress line and prints a message above it.
+
+        The progress bar is then redrawn on the next line.
+        """
+        if not _TTY or IsTraceToStderr() or self._quiet:
+            return
+
+        # Erase the current line, print the message with a newline,
+        # and then immediately redraw the progress bar on the new line.
+        sys.stderr.write("\r" + CSI_ERASE_LINE)
+        sys.stderr.write(msg + "\n")
+        sys.stderr.flush()
+        self.update(inc=0)
+
     def end(self):
         self._update_event.set()
         if not _TTY or IsTraceToStderr() or self._quiet: