streamclone: refactor code for deciding to stream clone Having this in a standalone function will eventually enable bundle2 to share code with the bundle1 code path. While I was here, I also added some comments to add clarity.

streamclone: move streaming clone logic from localrepo This is the last remnants of streaming clone code in localrepo.py. This is a mostly mechanical transplant of code to a new file. Only a rewrite of "self" to "repo" was performed. The code will be significantly refactored in upcoming patches. So don't scrutinize it too closely.