Iterable ObjectStorage interface for use in Repository struct

[JoshuaSjoding]

The current implementation of the Repository struct includes a pointer to core.RAWObjectStorage. This seems inappropriate because it prevents the development of alternative implementations of ObjectStorage. I presume that the direct reference was included because it provided convenient and fast access to the commit objects.

This pull request proposes the addition of one function to the ObjectStorage interface: IterType. This function returns an iterator of objects matching the given object type. The Repository is modified to make use of this new function, and RAWObjectStorage is modified to provide it.

The return value of IterType is a new interface called ObjectIter. Two implementations of ObjectIter are provided:

• ObjectLookupIter, which iterates over a slice of object hashes and looks up the given objects as needed. This is used to iterate through parent commits, for example.
• ObjectSliceIter, which iterates over a slice of objects.

Additional implementations could easily be added.

Both CommitIter and TreeIter are modified to make use of the new ObjectIter interface, as is Commit.Parents(). Note that this does away with the previous channel-based iterators.

The proposed implementation of RAWObjectStorage.IterType simply flattens the map of the requested type into a slice on the fly. This is probably fine for small to medium repositories, but might cause trouble for large repositories or if it's called frequently. Further optimization should be done as warranted.

Note that this pull request modifies several public types. As such it should probably be considered breaking for v2; it may be more appropriate to pull this instead into a v3 branch. I expect that other breaking changes will be needed to meet the needs of #19, and I suspect that all such work should be carried out in a branch until it's ready to be rolled out formally as v3. Feedback on this is most welcome.

[mcuadros]

I prefer call this method just Iter, is less verbose and clear enough

[mcuadros]

@JoshuaSjoding great job, we will merge this as soon as the minor things I commented get fixed. You can keep the PR to master, because we are using gopkg.in, is not a problem having an unreleased master.

Thanks!

[JoshuaSjoding]

@mcuadros I think that I've addressed all of your comments with the latest commit.

CommitIter has some decent test coverage now. Tree probably needs some work and should use a FileIter to be consistent, but I'll handle that with a separate issue and PR if it's wanted.

I also merged in the latest work from src-d:master for convenience. Let me know if you'd prefer that I not do that in the future.

[codecov-io]

Current coverage is 58.51%

Merging #20 into master will decrease coverage by -2.97% as of 99f8359

@@            master     #20   diff @@
======================================
  Files           24      24       
  Stmts         1532    1567    +35
  Branches       203     200     -3
  Methods          0       0       
======================================
- Hit            942     917    -25
- Partial         92      93     +1
- Missed         498     557    +59

Review entire Coverage Diff as of 99f8359

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[JoshuaSjoding]

@mcuadros I went ahead and updated the ObjectStorage functions New, Get and Set so that they can return errors now.