mergetool's test for rerere seems to be incorrect

Hi,

Occasionally when I use mergetool in a conflicted state ('git status'
says it's a conflict), nothing happens.

By bisection I found that bb0a484e985ef8d9bbbbeb172b1fcf4982634bef is
the offending commit.

I have rerere.enabled unset, but I did find an empty file MERGE_RR in
.git/, which explains my problem: mergetool chooses to use rerere
because it finds the MERGE_RR file, but rerere just exits silently
because it's not enabled.

Wouldn't it make sense to restrict mergetool's use of rerere to the
case when rerere.enabled is set?

I'm also a little confused about why the MERGE_RR file appeared there
in the first place. I can see that empty MERGE_RR files, with
different timestamps, have ended up in various other of my git
repositories as well. I never used rerere in these repos, and the only
non-git tools I have used for version control are stgit and kdiff3.

Erik