send-email: add --confirm option and configuration setting

send-email: add --confirm option and configuration setting

send-email violates the principle of least surprise by automatically
cc'ing additional recipients without confirming this with the user.

This patch teaches send-email a --confirm option. It takes the
following values:

 --confirm=always   always confirm before sending
 --confirm=never    never confirm before sending
 --confirm=cc       confirm before sending when send-email has
                    automatically added addresses from the patch to
                    the Cc list
 --confirm=compose  confirm before sending the first message when
                    using --compose. (Needed to maintain backwards
                    compatibility with existing behavior.)
 --confirm=auto     'cc' + 'compose'

If sendemail.confirm is unconfigured, the option defaults to 'compose'
if any suppress-Cc related options have been used, otherwise it defaults
to 'auto'.

Unfortunately, it is impossible to introduce this patch such that it
helps new users without potentially annoying some existing users. We
attempt to mitigate the latter by:

 * Allowing the user to set 'git config sendemail.confirm never'
 * Allowing the user to say 'all' after the first prompt to not be
   prompted on remaining emails during the same invocation.
 * Telling the user about the 'sendemail.confirm' setting if it is
   unconfigured whenever we prompt due to Cc before sending.
 * Only prompting if no --suppress related options have been passed, as
   using such an option is likely to indicate an experienced send-email
   user.

There is a slight fib in message informing the user of the
sendemail.confirm setting and this is intentional. Setting 'auto'
differs from leaving sendemail.confirm unset in two ways: 1) 'auto'
obviously squelches the informational message; 2) 'auto' prompts when
the Cc list has been expanded even in the presence of a --suppress
related option, where leaving sendemail.confirm unset does not. This is
intentional to keep the message simple, and to avoid adding another
sendemail.confirm value ('auto-except-suppress'?).

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>