Move MERGE_RR from .git/rr-cache/ into .git/

Move MERGE_RR from .git/rr-cache/ into .git/

If you want to reuse the rerere cache in another repository, and set
a symbolic link to it, you do not want to have the two repositories
interfer with each other by accessing the _same_ MERGE_RR.

For example, if you use contrib/git-new-workdir to set up a second
working directory, and you have a conflict in one working directory,
but commit in the other working directory first, the wrong "resolution"
will be recorded.

The easy solution is to move MERGE_RR out of the rr-cache/ directory,
which also corresponds with the notion that rr-cache/ contains cached
resolutions, not some intermediate temporary states.

Noticed by Kalle Olavi Niemitalo.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>