git-p4: fix freezing while waiting for fast-import progress

As part of its importing process, git-p4 sends a `checkpoint` followed
immediately by `progress` to fast-import to force synchronization.
Due to buffering, it is possible for the `progress` command to not be
flushed before git-p4 begins to wait for the corresponding response.
This causes the script to freeze completely, and is consistently
observable at least on python-3.6.9.

Make sure this command sequence is completely flushed before waiting.

Signed-off-by: Yang Zhao <yang.zhao@skyboxlabs.com>
Reviewed-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -2641,6 +2641,7 @@ def __init__(self):
     def checkpoint(self):
         self.gitStream.write("checkpoint\n\n")
         self.gitStream.write("progress checkpoint\n\n")
+        self.gitStream.flush()
         out = self.gitOutput.readline()
         if self.verbose:
             print("checkpoint finished: " + out)