Install the default "master" branch configuration after cloning a void

Install the default "master" branch configuration after cloning a void

After "cloning from an empty repository", we have a configuration to
describe the remote's URL and the default ref mappings, but we lack the
branch configuration for the default branch we create on our end,
"master".

It is likely that the empty repository we cloned from will point the
default "master" branch with its HEAD, so prepare the local configuration
to match.

Signed-off-by: Junio C Hamano <gitster@pobox.com>