config.c: rename "struct config_source cf"

config.c: rename "struct config_source cf"

The "cf" name is a holdover from before 4d8dd1494e (config: make parsing
stack struct independent from actual data source, 2013-07-12), when the
struct was named config_file. Since that acronym no longer makes sense,
rename "cf" to "cs". In some places, we have "struct config_set cs", so
to avoid conflict, rename those "cs" to "set" ("config_set" would be
more descriptive, but it's much longer and would require us to rewrap
several lines).

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>