http.c: add http.sslCertPasswordProtected option

http.c: add http.sslCertPasswordProtected option

Add a configuration option, http.sslCertPasswordProtected, and associated
environment variable, GIT_SSL_CERT_PASSWORD_PROTECTED, to enable SSL client
certificate password prompt from within git.  If this option is false and
if the environment variable does not exist, git falls back to OpenSSL's
prompts (as in earlier versions of git).

The environment variable may only be used to enable, not to disable
git's password prompt.  This behavior mimics GIT_NO_VERIFY; the mere
existence of the variable is all that is checked.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>