builtin/ls-remote: initialize repository based on fetch

builtin/ls-remote: initialize repository based on fetch

ls-remote may or may not operate within a repository, and as such will
not have been initialized with the repository's hash algorithm.  Even if
it were, the remote side could be using a different algorithm and we
would still want to display those refs properly.  Find the hash
algorithm used by the remote side by querying the transport object and
set our hash algorithm accordingly.

Without this change, if the remote side is using SHA-256, we truncate
the refs to 40 hex characters, since that's the length of the default
hash algorithm (SHA-1).

Note that technically this is not a correct setting of the repository
hash algorithm since, if we are in a repository, it might be one of a
different hash algorithm from the remote side.  However, our current
code paths don't handle multiple algorithms and won't for some time, so
this is the best we can do.  We rely on the fact that ls-remote never
modifies the current repository, which is a reasonable assumption to
make.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>