git-mergetool reverse file ordering

Hi,

Attached is a potential patch for reversing the order on which
git-mergetool presents the files to merge.

Currently, when running git-mergetool, it performs a sort of the files
to merge by alphabetical ordering. When working on C, this has the
annoying effect of presenting the merge for a .c* files before the
header files; which is always a bit harder to do. Reading the header
first to figure out what the other dude changed is usually preferred.

The attach patch reverse the order (-r flag to sort) so *.h* are
merged before  *.c* files

PS, given the simplicity of the patch, I have not tested it.

Regards

Luis