Set core.precomposeunicode to true on e.g. HFS+

Set core.precomposeunicode to true on e.g. HFS+

When core.precomposeunicode was introduced in 76759c7d,
it was set to false on a unicode decomposing file system like HFS+
to be compatible with older versions of Git.

The Mac OS users need to find out that this configuration exist
and change it manually from false to true.

A smoother workflow can be achieved,
so set core.precomposeunicode to true on a decomposing file system.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>