IndexDiff/SubmoduleWalk: handle submodule.<name>.ignore correctly

IndexDiff/SubmoduleWalk: handle submodule.<name>.ignore correctly

IndexDiff would apply ignore mode ALL from .gitmodules to all remaining
submodules, and would ignore other settings from .gitignore and always
apply the setting defined on the IndexDiff instead. Correct that.

In canonical git the ignore setting from .gitmodules can also be
overridden by .git/config.[1] Implement that override in SubmoduleWalk.

[1] https://git-scm.com/docs/gitmodules#Documentation/gitmodules.txt-submoduleltnamegtignore

Bug: 521613
Change-Id: I9199fd447e41c7838924856dce40678370b66395
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>