imap-send.c: support GIT_CURL_VERBOSE

imap-send.c: support GIT_CURL_VERBOSE

When using git-imap-send to send via cURL, support setting
the GIT_CURL_VERBOSE environment variable to enable cURL's
verbose mode.

The existing http.c code already supports this and does
it by simply checking to see whether or not the environment
variable exists -- it does not examine the value at all.

For consistency, enable CURLOPT_VERBOSE when GIT_CURL_VERBOSE
is set by using the exact same test that http.c does.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>