config: initialize opts structure in repo_read_config()

config: initialize opts structure in repo_read_config()

Initialize opts structure in repo_read_config().

This change fixes a crash in later commit after a new field is added
to the structure.

In commit 3b256228a66f8587661481ef3e08259864f3ba2a, repo_read_config()
was added.  It only initializes 3 fields in the opts structure.  It is
passed to config_with_options() and then to do_git_config_sequence().
However, do_git_config_sequence() drops the opts on the floor and calls
git_config_from_file() rather than git_config_from_file_with_options(),
so that may be why this hasn't been a problem in the past.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>