[PATCH] Add the merge test Linus called "test script from hell".

[PATCH] Add the merge test Linus called "test script from hell".

This is an adaptation to the test framework of a historic test
that was used before three way merge form of read-tree was
introduced, and subsequently used to validate the read-tree -m
merge works correctly.  It covers all the tricky cases known
back then and also have been updated to cover conflicting
files/directories cases since then.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Petr Baudis <pasky@ucw.cz>