move git_getpass to its own source file

This is currently in connect.c, but really has nothing to
do with the git protocol itself. Let's make a new source
file all about prompting the user, which will make it
cleaner to refactor.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

imap-send.c

@@ -25,6 +25,7 @@
 #include "cache.h"
 #include "exec_cmd.h"
 #include "run-command.h"
+#include "prompt.h"
 #ifdef NO_OPENSSL
 typedef void *SSL;
 #else