diff: submodule inline diff to initialize env array.

diff: submodule inline diff to initialize env array.

David reported:
> When I try to run `git diff --submodule=diff` in a submodule which has
> it's own submodules that have changes I get the error: fatal: bad
> object.

This happens, because we do not properly initialize the environment
in which the diff is run in the submodule. That means we inherit the
environment from the main process, which sets environment variables.
(Apparently we do set environment variables which we do not set
when not in a submodules, i.e. the .git directory is linked)

This commit, just like fd47ae6a5b (diff: teach diff to display
submodule difference with an inline diff, 2016-08-31) introduces bad
test code (i.e. hard coded hash values), which will be cleanup up in
a later patch.

Reported-by: David Parrish <daveparrish@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>