editorconfig: indicate settings should be kept in sync

editorconfig: indicate settings should be kept in sync

Now that we have two places where we set code formatting settings,
.editorconfig and .clang-format, it's possible that they could fall out
of sync.  This is relatively unlikely, since we're not likely to change
the tab width or our preference for tabs, but just in case, add comments
to both files reminding future developers to keep them in sync.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>