config: restructure format_config()

config: restructure format_config()

The recent changes have replaced the bodies of most if/else-if cases
with simple helper method calls. This makes it easy to adapt the
structure into a clearer switch statement, leaving a simple if/else in
the default case.

Make things a little simpler to read by reducing the nesting depth via a
new goto statement when we want to skip values.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>