send-email: Add config option for sender address

send-email: Add config option for sender address

The sender address, as specified with the '--from' command line option,
couldn't be set in the config file.  So add a new config option,
'sendemail.from', which sets it.  One can use 'sendemail.<identity>.from'
as well of course, which is likely the more useful case.

The sender address would default to GIT_AUTHOR_IDENT, which is usually the
right thing, but this doesn't allow switching based on the identity
selected.  It's possible to switch the SMTP server and envelope sender by
using the '--identity' option, in which case one probably wants to use a
different from address as well, but this had to be manually specified.

The documentation for 'from' is also corrected somewhat.  If '--from' is
specified (or the new sendemail.from option is used) then the user isn't
prompted.  The default with no '--from' option (or sendemail.from option)
is GIT_AUTHOR_IDENT first then GIT_COMMITTER_IDENT, not just
GIT_COMMITTER_IDENT.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>