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>

Documentation/git-send-email.txt

@@ -69,9 +69,12 @@ and In-Reply-To headers will be used unless they are removed.
 Missing From or In-Reply-To headers will be prompted for.
 
 --from=<address>::
-	Specify the sender of the emails.  This will default to
-	the value GIT_COMMITTER_IDENT, as returned by "git var -l".
-	The user will still be prompted to confirm this entry.
+	Specify the sender of the emails.  If not specified on the command line,
+	the value of the 'sendemail.from' configuration option is used.  If
+	neither the command line option nor 'sendemail.from' are set, then the
+	user will be prompted for the value.  The default for the prompt will be
+	the value of GIT_AUTHOR_IDENT, or GIT_COMMITTER_IDENT if that is not
+	set, as returned by "git var -l".
 
 --in-reply-to=<identifier>::
 	Specify the contents of the first In-Reply-To header.

git-send-email.perl

@@ -210,6 +210,7 @@ my %config_settings = (
     "envelopesender" => \$envelope_sender,
     "multiedit" => \$multiedit,
     "confirm"   => \$confirm,
+    "from" => \$sender,
 );
 
 # Handle Uncouth Termination