feature: store interactive editor content for revisiting it (after failures)

[elonderin]

Unless i just haven found that file this feature i have been missing:

when revise tries to do its thing it sometimes fails which is mostly due to botching the editing. Hence it would be grand not loose all that work and retry with where one has left off. 2 ideas come to mind on this:

a) echo where the content is stored (e.g. $PWD/.revise.txt, or .git/info/revise.txt)
b) have an option to rerun the last interactive session with the editor prefilled with that content + afterwards the normal generated content but commented out, so that it may serve as a reference.

This could also help in the case of #34 in which the revise would run thru,but w/o a failure. Nevertheless, those precious message edits are lost to /dev/null.