mv: allow moving nested submodules

mv: allow moving nested submodules

When directories are moved using `git mv` all files in the directory
have been just moved, but no further action was taken on them. This
was done by assigning the mode = WORKING_DIRECTORY to the files
inside a moved directory.

submodules however need to update their link to the git directory as
well as updates to the .gitmodules file. By removing the condition of
`mode != INDEX` (the remaining modes are BOTH and WORKING_DIRECTORY) for
the required submodule actions, we perform these for submodules in a
moved directory.

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