grep: fix race conditions at grep_submodule()

grep: fix race conditions at grep_submodule()

There're currently two function calls in builtin/grep.c:grep_submodule()
which might result in race conditions:

- submodule_from_path(): it has config_with_options() in its call stack
  which, in turn, may have read_object_file() in its own. Therefore,
  calling the first function without acquiring grep_read_mutex may end
  up causing a race condition with other object read operations
  performed by worker threads (for example, at the fill_textconv()
  call in grep.c:fill_textconv_grep()).
- parse_object_or_die(): it falls into the same problem, having
  repo_has_object_file(the_repository, ...) in its call stack. Besides
  that, parse_object(), which is also called by parse_object_or_die(),
  is thread-unsafe and also called by object reading functions.

It's unlikely to really fall into a data race with these operations as
the volume of calls to them is usually very low. But we better protect
ourselves against this possibility, anyway. So, to solve these issues,
move both of these function calls into the critical section of
grep_read_mutex.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>