git-send-email.perl: Add --to-cmd

Add the ability to use a command line --to-cmd=cmd
to create the list of "To:" addresses.

Used a shared routine for --cc-cmd and --to-cmd.

Did not use IPC::Open2, leaving that for Ævar if
ever he decides to fix the other bugs he might find.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-send-email.txt b/Documentation/git-send-email.txt
index c283084..fff97a3 100644
--- a/Documentation/git-send-email.txt
+++ b/Documentation/git-send-email.txt
@@ -97,7 +97,7 @@
 	Specify the primary recipient of the emails generated. Generally, this
 	will be the upstream maintainer of the project involved. Default is the
 	value of the 'sendemail.to' configuration value; if that is unspecified,
-	this will be prompted for.
+	and --to-cmd is not specified, this will be prompted for.
 +
 The --to option must be repeated for each user you want on the to list.
 
@@ -177,6 +177,12 @@
 Automating
 ~~~~~~~~~~
 
+--to-cmd=<command>::
+	Specify a command to execute once per patch file which
+	should generate patch file specific "To:" entries.
+	Output of this command must be single email address per line.
+	Default is the value of 'sendemail.tocmd' configuration value.
+
 --cc-cmd=<command>::
 	Specify a command to execute once per patch file which
 	should generate patch file specific "Cc:" entries.