stage: add missing flush before project prompt

Bug: https://crbug.com/gerrit/13223
Change-Id: Ib279d86a52e1035e02d6f7d8f053c3a43e721032
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343555
Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/stage.py

@@ -75,6 +75,7 @@ The '%prog' command stages files to prepare the next commit.
       out.nl()
 
       out.prompt('project> ')
+      out.flush()
       try:
         a = sys.stdin.readline()
       except KeyboardInterrupt: