Expose merge analysis

[carlosmn]

This allows us to ask the library about what kind of actions
are available for merging into HEAD.

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I'm still not quite sure what the overloads should be. Maybe leave as-is and think about exposing annotated commits as a type.

[ethomson]

Yeah, I worry about having these two overloads. Because what happens when you want to octopus merge two commits and a ref? (Well, this is entirely theoretical, since libgit2 doesn't even do octopus merging, but you know.)

I wonder if Commit and Reference should implement IMergeable or something?

[carlosmn]

You can peel the Reference to a commit, that shouldn't be much of an issue. They could implement IAnnotatable or something since what we actually want is an annotated commit and these are just convenience methods for that.

[ethomson]

You can peel the Reference to a commit, that shouldn't be much of an issue.

Sure - I guess my thinking here is twofold: first, it would be nice if we didn't make users do the peeling. :) And second, Merge will behave differently when handed a commit versus an actual branch. Merge analysis, of course, will not but I'm wondering if we can make sure these APIs are nice and similar. At the moment we have:

    MergeResult Merge(Commit commit, Signature merger, MergeOptions options);
    MergeResult Merge(Branch branch, Signature merger, MergeOptions options);
    MergeResult Merge(string committish, Signature merger, MergeOptions options);

And it would definitely be nice to get rid of these overloads and have a single Merge that takes the same type of commits/references/etc that the analysis method takes...

[carlosmn]

Sure, we can accept a IAnnotateCommit that knows how to get the actual handle we want from a branch/ref/revparse. It's a shame you can't implement it for string but we can have two overloads, one for rev-parse strings and one for objects.

[carlosmn]

Updated with more interfaces, yay. The CI error seems to come from us assuming interfaces are only ever there for mocking, which seems like a bad idea.