Merge branch 'tb/precompose-autodetect-fix'

On MacOS X, we detected if the filesystem needs the "pre-composed
unicode strings" workaround, but did not automatically enable it.
Now we do.

* tb/precompose-autodetect-fix:
Set core.precomposeunicode to true on e.g. HFS+

diff --cc t/t7400-submodule-basic.sh
Simple merge