Ensure builtin-fetch honors {fetch,transfer}.unpackLimit

Ensure builtin-fetch honors {fetch,transfer}.unpackLimit

The only way to configure the unpacking limit is currently through
the .git/config (or ~/.gitconfig) mechanism as we have no existing
command line option interface to control this threshold on a per
invocation basis.  This was intentional by design as the storage
policy of the repository should be a repository-wide decision and
should not be subject to variations made on individual command
executions.

Earlier builtin-fetch was bypassing the unpacking limit chosen by
the user through the configuration file as it did not reread the
configuration options through fetch_pack_config if we called the
internal fetch_pack() API directly.  We now ensure we always run the
config file through fetch_pack_config at least once in this process,
thereby setting our unpackLimit properly.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>