stream-clone: add a v3 version of the protocol
This new version is less rigid regarding the extract number of files and number
of bytes to be actually transfered, it also lays the groundwork for other
improvements.
The format stays experimental, but this is an interesting base to build upon.
Set up
$ hg init repo
$ cd repo
$ echo a > a
$ hg add a
$ hg commit -m a
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate