Ref T8259. Currently, arc upload uses new logic but arc diff uses older logic internally. This prevents arc diff from uploading files larger than 4MB to newer servers.

Split the upload logic apart so the two upload pathways can share it. Callers now build a list of FileDataRefs and hand them to an Uploader for upload.

Ran arc upload on:

• One file.
• Several files.
• Small files.
• Big files.
• Directories.
• Unreadable files.
• Files full of random data.
• The same file over and over again.
• The same big file over and over again.
• Artificially broke file.allocate and redid some of the simple cases (large/chunked aren't expected to work in this case).