Expose setting workdir and detatched HEADs

[oconnor663]

A couple new features that I need for a tool I'm working on (https://github.com/oconnor663/peru). I'm totally new to libgit2, so I'm sure I made mistakes here.

The next thing I need to expose is git_merge_trees. I'd appreciate any advice for where that belongs.

[oconnor663]

I think this failure repros in master with the latest updates from libgit2.

[jdavid]

Hello, please rebase or merge against master, there are conflicts since we moved to libgit2 v0.21.0

Otherwise it looks good, just the second patch does not respect the 4 spaces indentation coding style.

[oconnor663]

Rebased and fixed the style. I noticed in the docs it mentions that all APIs that take an Oid should also accept a hex string. What's the right way to make this work for Repository_head__set__?

[jdavid]

Regarding git_merge_trees, I don't know much about that side of the code, maybe @carlosmn wants to comment.

[carlosmn]

git_merge_trees is a fairly low-level operation, which doesn't necessarily belong anywhere in particular. Since its first input parameter is the repository, we could hang if off there as Repository.merge_trees(), but it's not really a repository operation, so it would fit just as well as pygit2.merge_trees() which takes in a Repository as its first argument.

[jdavid]

If it takes a repository, then I vote for Repository.merge_trees()

[oconnor663]

I actually ended up going with TreeBuilder and manually merging things, because what I needed was more like the "refuse to overwrite entries that already existed in the original index file" behavior of git read-tree --prefix. I couldn't see a way to specify different merge algorithms for merge_trees. Any chance I missed something?