When the stream writing function was written, it assume that
libssh2_channel_write() would always write all of the data to the
wire. This is only true for the first 32k of data, which it tries to
fit into one ssh packet.

Since it can perform short writes, call it in a loop like we do for
send(), advancing the buffer offset.

We want to check whether the credentials callback is NULL, not whether
the payload is.

Whe already worked out the kinks with the function used in the local
transport. Expose it and make use of it in the local clone method
instead of trying to work it out again.

We have too many places where we repeat free code, so when adding the
new free to the generic code, it didn't take for the local transport.

While there, fix a C99-ism that sneaked through.

Add support for the symref extension

When using the local transport, we always have the symbolic information
available, so fill it.

Add a symref_target field to git_remote_head to expose the symref
mappings to the user.

The protocol has a capability which allows the server to tell us which
refs are symrefs, so we can e.g. know which is the default branch.

This capability is different from the ones we already support, as it's
not setting a flag to true, but requires us to store a list of
refspec-formatted mappings.

This commit does not yet expose the information in the reference
listing.