send-email: enable copying emails to IMAP folder without actually sending them

send-email: enable copying emails to IMAP folder without actually sending them

`git imap-send` was built on the idea of copying emails to an IMAP folder
like drafts, and sending them later using an email client. Currently
the only way to do it is by piping output of `git format-patch` to IMAP
send.

Add another way to do it by using `git send-email` with the
`--use-imap-only` or `sendmail.useImapOnly` option. This allows users to
use the advanced features of `git send-email` like tweaking Cc: list
programmatically, compose the cover letter, etc. and then send the well
formatted emails to an IMAP folder using `git imap-send`.

While at it, use `` instead of '' for --smtp-encryption ssl in help
section of `git send-email`.

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