imap-send: display port alongwith host when git credential is invoked

imap-send: display port alongwith host when git credential is invoked

When requesting for passsword, git credential helper used to display
only the host name. For example:

    Password for 'imaps://gargaditya08%40live.com@outlook.office365.com':

Now, it will display the port along with the host name:

    Password for 'imaps://gargaditya08%40live.com@outlook.office365.com:993':

This has been done to make credential helpers more specific for ports.
Also, this behaviour will also mimic git send-email, which displays
the port along with the host name when requesting for a password.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>