git-p4: rewrite prompt to be Windows compatible

git-p4: rewrite prompt to be Windows compatible

The existing function prompt(prompt_text) does not work correctly when
run on Windows 10 bash terminal when launched from the sourcetree
GUI application. The stdout is not flushed properly so the prompt text
is not displayed to the user until the next flush of stdout, which is
quite confusing.

Change this method by:
* Adding flush to stderr, stdout, and stdin
* Use readline from sys.stdin instead of raw_input.

The existing strip().lower() are retained.

Signed-off-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>