grep: search history of moved submodules

If a submodule was renamed at any point since it's inception then if you
were to try and grep on a commit prior to the submodule being moved, you
wouldn't be able to find a working directory for the submodule since the
path in the past is different from the current path.

This patch teaches grep to find the .git directory for a submodule in
the parents .git/modules/ directory in the event the path to the
submodule in the commit that is being searched differs from the state of
the currently checked out commit.  If found, the child process that is
spawned to grep the submodule will chdir into its gitdir instead of a
working directory.

In order to override the explicit setting of submodule child process's
gitdir environment variable (which was introduced in '10f5c526')
`GIT_DIR_ENVIORMENT` needs to be pushed onto child process's env_array.
This allows the searching of history from a submodule's gitdir, rather
than from a working directory.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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