Allow for selection of merge base if not unique

[scruff311]

Allow user to select commit to use for merge base in there is not a unique merge base.

[lucaswerkmeister]

A cancel option might be useful, I think.

(Also, for the record, implements #52.)

[mhagger]

Cool, thanks for working on this!

I like the idea a lot, and more than one user has stumbled over this problem so it will be a welcome feature.

I was a bit surprised that you implemented this by asking the user to choose one of the merge bases interactively. There are only very few git commands that have an interactive component, so this approach was unexpected. It would also effectively make git-imerge unusable from a script.

I think my preference would be for the program to emit an error message listing the merge bases (which you have already implemented) but then just abort. And for there to be a new command-line option like --merge-base=SHA1 to let the user specify an explicit merge base at his next attempt. It is a little bit more work for the user, but it is more consistent with the rest of the interface (and with that of Git itself).