http: use credential API to get passwords

http: use credential API to get passwords

This patch converts the http code to use the new credential
API, both for http authentication as well as for getting
certificate passwords.

Most of the code change is simply variable naming (the
passwords are now contained inside the credential struct)
or deletion of obsolete code (the credential code handles
URL parsing and prompting for us).

The behavior should be the same, with one exception: the
credential code will prompt with a description based on the
credential components. Therefore, the old prompt of:

  Username for 'example.com':
  Password for 'example.com':

now looks like:

  Username for 'https://example.com/repo.git':
  Password for 'https://user@example.com/repo.git':

Note that we include more information in each line,
specifically:

  1. We now include the protocol. While more noisy, this is
     an important part of knowing what you are accessing
     (especially if you care about http vs https).

  2. We include the username in the password prompt. This is
     not a big deal when you have just been prompted for it,
     but the username may also come from the remote's URL
     (and after future patches, from configuration or
     credential helpers).  In that case, it's a nice
     reminder of the user for which you're giving the
     password.

  3. We include the path component of the URL. In many
     cases, the user won't care about this and it's simply
     noise (i.e., they'll use the same credential for a
     whole site). However, that is part of a larger
     question, which is whether path components should be
     part of credential context, both for prompting and for
     lookup by storage helpers. That issue will be addressed
     as a whole in a future patch.

Similarly, for unlocking certificates, we used to say:

  Certificate Password for 'example.com':

and we now say:

  Password for 'cert:///path/to/certificate':

Showing the path to the client certificate makes more sense,
as that is what you are unlocking, not "example.com".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>