send-email: don't create temporary compose file until it is needed

send-email: don't create temporary compose file until it is needed

Commit eed6ca7 caused a minor regression when it switched to using
tempfile() to generate the temporary compose file. Since tempfile()
creates the file at the time it generates the filename, zero-length
temporary files are being left behind unless --compose is used (in which
case the file is cleaned up).

This patch fixes the regression by not calling tempfile() to generate
the compose filename unless --compose is in use.

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