grep: remove "repo" arg from non-supporting funcs

grep: remove "repo" arg from non-supporting funcs

As part of commit f9ee2fcdfa ("grep: recurse in-process using 'struct
repository'", 2017-08-02), many functions in builtin/grep.c were
converted to also take "struct repository *" arguments. Among them were
grep_object() and grep_objects().

However, at least grep_objects() was converted incompletely - it calls
gitmodules_config_oid(), which references the_repository.

But it turns out that the conversion was extraneous anyway - there has
been no user-visible effect - because grep_objects() is never invoked
except with the_repository. This is because grepping through objects
cannot be done recursively into submodules.

Revert the changes to grep_objects() and grep_object() (which conversion
is also extraneous) to show that both these functions do not support
repositories other than the_repository.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>