1. Fix sort order problem with submodules where "mod" was sorting
after "mod-plus" because they were being sorted as "mod/" and
"mod-plus/". This involved pushing the "contains a .git entry"
test significantly lower in the stack.
2. Reinstate behavior that a directory which contains a .git entry
will be treated as a submodule during iteration even if it is
not yet added to the .gitmodules.
3. Now that any directory containing .git is reported as submodule,
we have to be more careful checking for GIT_EEXISTS when we
do a submodule lookup, because that is the error code that is
returned by git_submodule_lookup when you try to look up a
directory containing .git that has no record in gitmodules or
the index.
This should confirm that issue #835 is fixed where a submodule that
is only declared in the .gitmodules file was not accessible via the
submodule APIs.
