This was whittled down to a compatibility wrapper around the more
flexible git_prompt() in 1cb0134f (refactor git_getpass into generic
prompt function, 2011-12-10), waiting for the final callers to go
away.  That happened in 791643a8 (imap-send: use git-credential,
2014-04-28) when imap-send learned to use the credential interface.
Reviewed-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>