[FEATURE-REQUEST] difftool --dir-diff: use the commit names as directory names instead of left/right

Hi.

Right now, when I use difftool --dir-diff, the temp dirs are creates as
e.g.:
/tmp/git-difftool.QqP8x/left
/tmp/git-difftool.QqP8x/right

Wouldn't it be nice, if instead of left/right... the specified commit
name would be used?

e.g.
/tmp/git-difftool.QqP8x/r1.1.1
/tmp/git-difftool.QqP8x/HEAD
or
/tmp/git-difftool.QqP8x/fd4e4005a4b7b3e638bc405be020b867f8881e72
/tmp/git-difftool.QqP8x/ce0747b74fccd20707b6f495068503e69e5473df

Cause then, one would see in the difftool which side is what, without
the need to remember how git difftool was invoked.


Of course one might probably need to escape the commit names if they
contain stuff like "/" or "..", etc.



Cheers,
Chris.