http.c: prompt for SSL client certificate password

http.c: prompt for SSL client certificate password

If an SSL client certificate is enabled (via http.sslcert or
GIT_SSL_CERT), prompt for the certificate password rather than
defaulting to OpenSSL's password prompt.  This causes the prompt to only
appear once each run.  Previously, OpenSSL prompted the user *many*
times, causing git to be unusable over HTTPS with client-side
certificates.

Note that the password is stored in memory in the clear while the
program is running.  This may be a security problem if git crashes and
core dumps.

The user is always prompted, even if the certificate is not encrypted.
This should be fine; unencrypted certificates are rare and a security
risk anyway.

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