parse: separate out parsing functions from config.h

parse: separate out parsing functions from config.h

The files config.{h,c} contain functions that have to do with parsing,
but not config.

In order to further reduce all-in-one headers, separate out functions in
config.c that do not operate on config into its own file, parse.h,
and update the include directives in the .c files that need only such
functions accordingly.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>