Bitbucket fetchDiffFiles() fetches file contents serially instead of in parallel

Created by: felixfbecker

This uses switchMap to fetch the data serially. It should be using zip() or forkJoin().

https://github.com/sourcegraph/sourcegraph/blob/2047b2bcbeeaa3474310b3ba21d4cb88c5aae772/client/browser/src/libs/bitbucket/file_info.ts#L27-L57