Recognize and report server-side error messages

When e.g. a repository isn't found, the server sends an error saying
so. Put that error message in our error buffer.

Trying to send every single line immediately won't give us any speed
improvement and duplicates the code we need for other transports. Make
the git transport use the same buffer functions as HTTP.

This changes the git_remote_download() API, but the existing one is
silly, so you don't get to complain.

The new API allows to know how much data has been downloaded, how many
objects we expect in total and how many we've processed.

This fixes all the warnings on win64 except those in deps, which
come from the regex code.

Adds a new public reference function `git_reference_lookup_oid`
that directly resolved a reference name to an OID without returning
the intermediate `git_reference` object (hence, no free needed).

Internally, this adds a `git_reference_lookup_resolved` function
that combines looking up and resolving a reference.  This allows
us to be more efficient with memory reallocation.

The existing `git_reference_lookup` and `git_reference_resolve`
are reimplmented on top of the new utility and a few places in the
code are changed to use one of the two new functions.