Restrict deep nesting of configfiles

There is no point in restricting the number of included config-
files, but there is a point in restricting the nestinglevel
of configfiles: to avoid recursive inclusions. This is easily
achieved by decrementing the static nesting-variable upon exit
from cgit_read_config().

Also fix some whitespace breakage.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>