builtin/grep.c: respect 'submodule.recurse' option

builtin/grep.c: respect 'submodule.recurse' option

In builtin/grep.c we parse the config before evaluating the command line
options. This makes the task of teaching grep to respect the new config
option 'submodule.recurse' very easy by just parsing that option.

As an alternative I had implemented a similar structure to treat
submodules as the fetch/push command have, including
* aligning the meaning of the 'recurse_submodules' to possible submodule
  values RECURSE_SUBMODULES_* as defined in submodule.h.
* having a callback to parse the value and
* reacting to the RECURSE_SUBMODULES_DEFAULT state that was the initial
  state.

However all this is not needed for a true boolean value, so let's keep
it simple. However this adds another place where "submodule.recurse" is
parsed.

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