status: reduce duplicated setup code

status: reduce duplicated setup code

We have three output formats: short, porcelain, and long.
The short and long formats respect user-config, and the
porcelain one does not. This led to us repeating
config-related setup code for the short and long formats.

Since the last commit, color config is explicitly cleared
when showing the porcelain format. Let's do the same with
relative-path configuration, which enables us to hoist the
duplicated code from the switch statement in cmd_status.

As a bonus, this fixes "commit --dry-run --porcelain", which
was unconditionally setting up that configuration, anyway.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>