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>