When editing in the middle of a rebase, dont clear on quit

[forivall]

Fixes #739

Also, sets up the fix for #685 by detecting when we're using git-revise, still need to finish that up and conditionally add the git-revise commands of index and cut. I might also want to add special behaviour for index to automatically move the entry to the end (see https://github.com/mystor/git-revise/issues/16 )

Review on the quality of my rust code greatly appreciated!