bundle2: clarify in docstring that header size is for a single header
The docstring for the header size field currently says "The total
number of Bytes used by the part headers", but the size is about a
single header, so let's change it to "header".
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 ..