bundle2: add a 'copy' method on parts
This is the first step in our quest for preserving the server output on error
(issue4594). We want to be able to copy the output parts from the aborted reply
into the exception bundle.
The function will be used in a later patch.
basic test for hg debugrebuildstate
$ hg init repo
$ cd repo
$ touch foo bar
$ hg ci -Am 'add foo bar'
adding bar
adding foo
$ touch baz
$ hg add baz
$ hg rm bar
$ hg debugrebuildstate
state dump after
$ hg debugstate --nodates | sort
n 644 -1 set bar
n 644 -1 set foo
status
$ hg st -A
! bar
? baz
C foo
$ cd ..