Updates to the previous mergetool patches

[Charles Bailey <charles@hashpling.org>]

These two patches go on top of 682b451f and replace the 'DONTMERGE'
commit: 09b511016. If it would be easier if I cooked these in a
different way then let me know.

I've implemented a replacement for the "keep going" option which is now
to prompt after a failed merge to see if the user wants to continue
merging the remaining paths. This gets rid of a need for an option of
configuration option.

I haven't implemented any change to the logic as to whether to pause
before invoking the merge tool based on which merge tool is being used.

The added tests for the return status of the merge are slightly less
neat than I would have liked, but I think are necessary. I test after
invoking the merge tool to make sure that the "roll-up" status is
correct, and before invoking the next tool to see if the user wants to
continue despite the failure of the previous merge.

Only testing after the merge tool invocation can end up with a "do you
want to continue" before mergetool has determined that there are, in
fact, no more files to merge. Only testing before the merge means that
the status of the last merge doesn't contribute to the script's return
value. I don't know how many people rely on the return value of git
mergetool, but why not keep it consistent?

The second patch makes optional the retention of temporary files on a
failed merge optional. It seperates out the behaviour change that I'd
forgotten I'd left in the previous version of the "-k" patch.