Add MergeCommitsIntoIndex to ObjectDatabase

[tyrielv]

Adds a new method MergeCommitsIntoIndex to ObjectDatabase to make the intermediate result of MergeCommits available (so that conflicts can be resolved directly against the produced index).

Also adds WriteToTree to Index so that the produced index can be converted to a tree just like MergeCommits currently does.

[ethomson]

I'm not wild about returning a new Index object whose lifecycle is tied to the Repository. This is fine for services that are dealing with pull requests, since they'll spin up a repository, do a merge, then dispose the repository (and this merge result) but it's not going to be ideal for (say) GUI tools or anything that does multiple in-memory merges on the same Repository.

I think that we should create an Index subclass that's IDisposeable and doesn't register with the Repository for being disposed. TransientIndex, for lack of a better name...

I briefly (and by briefly, I mean I spent way too long on it) thought about making Index itself disposeable and replaceable, so that you could swap in a new index on the repository. I don't think that this is a path to success in the near term and that we should just have one type that represents the repository index and one that represents a transient index that's returned by merge / rebase / revert / etc / etc.

[ethomson]

Thanks - it looks like the appveyor build is failing because of the GitHub TLS 1.2 upgrades 🙄 . (It's the gift that keeps on giving!)

I'll push an update and I'll merge this when it's fixed.

Thanks for this work!

[ethomson]

Thanks for working on this!