Read .gitmodules config from the tree in SubmoduleWalk

It is not always appropriate to use the .gitmodules file from the
working tree, for example if reading the modules at a specific commit.
And sometimes it is impossible, as in a bare repository.

When using the static factory methods, automatically set up the
appropriate root tree so lazy loading of the config file reads from
the appropriate place. Leave the current behavior of looking in the
working tree as a fallback for the case where walking the index.

Change-Id: I71b7ed3ba16c80b0adb8c5fd85b5c37fd4aef8eb
4 files changed