Refactor allCommits to return CommitIterator instead,

as loading all commits in one go is not good for
repository with a lot of commits

SwiftGit2/Repository.swift

@@ -540,4 +540,64 @@ final public class Repository {
 		}
 		return Result.success(commits)
 	}
+	
+	public class CommitIterator: IteratorProtocol {
+		public typealias Element = Result<Commit, NSError>
+		var repo: Repository
+		var branch: Branch
+		var revisionWalker: OpaquePointer? = nil
+		var oid: git_oid
+		
+		public init(repo: Repository, branch: Branch) {
+			self.repo = repo
+			self.branch = branch
+			self.oid = branch.oid.oid
+			setupRevisionWalker()
+		}
+		
+		deinit {
+			git_revwalk_free(self.revisionWalker)
+		}
+		
+		private func setupRevisionWalker() {
+			git_revwalk_new(&revisionWalker, repo.pointer)
+			git_revwalk_sorting(revisionWalker, GIT_SORT_TOPOLOGICAL.rawValue)
+			git_revwalk_sorting(revisionWalker, GIT_SORT_TIME.rawValue)
+			git_revwalk_push(revisionWalker, &oid)
+		}
+		
+		private func error(from gitCommands: [(key: String, value: () -> Int32)]) -> NSError? {
+			// TODO: Make a  flatMap command that stops on first nil
+			let errors: [NSError] = gitCommands.flatMap {
+				let result = $0.value()
+				return result == GIT_OK.rawValue ? nil : NSError(gitError: result, pointOfFailure: $0.key)
+			}
+			if errors.count > 0 {
+				return errors.first!
+			} else {
+				return nil
+			}
+		}
+		
+		public func next() -> Element? {
+			var unsafeCommit: OpaquePointer? = nil
+			let gitCommands = [
+				(key: "git_revwalk_next", value: { return git_revwalk_next(&self.oid, self.revisionWalker) }),
+				(key: "git_commit_lookup", value: { return git_commit_lookup(&unsafeCommit, self.repo.pointer, &self.oid) })
+			]
+			if let error = error(from: gitCommands) {
+				return Result.failure(error)
+			} else {
+				guard let commit = unsafeCommit else {
+					return nil
+				}
+				git_commit_free(unsafeCommit)
+				return Result.success(Commit(commit))
+			}
+		}
+	}
+	
+	public func commits(in branch: Branch) -> CommitIterator {
+		return CommitIterator(repo: self, branch: branch)
+	}
 }