git-send-email: ssh/login style password requests

git-send-email: ssh/login style password requests

Whilst convenient, it is most unwise to record passwords
in any place but one's brain. Moreover, it is especially
foolish to store them in configuration files, even with
access permissions set accordingly.

git-send-email has been amended, so that if it detects
an smtp username without a password, it promptly prompts
for the password and masks the input for privacy.

Furthermore, the argument to --smtp-pass has been rendered
optional.

The documentation has been updated to reflect these changes.

Signed-off-by: Michael Witten <mfwitten@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>