grep_source_init() can create "struct grep_source" objects and,
depending on the value of the type passed, some void-pointer parameters have
different meanings. Because one of these types (GREP_SOURCE_OID) will
require an additional parameter in a subsequent patch, take the
opportunity to increase clarity and type safety by replacing this
function with individual functions for each type.
Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br> Signed-off-by: Junio C Hamano <gitster@pobox.com>