checkout: respect diff.ignoreSubmodules setting

checkout: respect diff.ignoreSubmodules setting

When 'git checkout' reports uncommitted changes, it also does so for
submodules.

The default mode is now to look really hard into submodules, not only
for different commits, but also for modified files. Since this can be
pretty expensive when there are a lot (and large) submodules, there is
the diff.ignoreSubmodules option.

Let's respect that setting when 'git checkout' reports the uncommitted
changes, since it does nothing else than a 'git diff --name-status'.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>