This is a contract that we made in the library and that we need to uphold. The
contents of a blob can never be NULL because several parts of the library (including
the filter and attributes code) expect `git_blob_rawcontent` to always return a
valid pointer.

Update message for error during push

Clarify the git_remote_ls() documentation

fetch: clear the connection data on close

travis: disable the cert callback on OSX

The sshd on OSX has stopped responding to the setting which this test
assumes is there, so let's disable it until we can figure out what's
happening.

When we fetch twice with the same remote object, we did not properly
clear the connection flags, so we would leak state from the last
connection.

This can cause the second fetch with the same remote object to fail if
using a HTTP URL where the server redirects to HTTPS, as the second
fetch would see `use_ssl` set and think the initial connection wanted to
downgrade the connection.

On disconnect we simply ask the transport to close the connection, we do
not free it.

Make it clear that this is not the ls-remote command but a way to access
the data we have and how long it's kept around.