bundle2: move the fromheader closure into the class itself
The class is now directly related to this header data. We can sanely move it on
the class.
I do not like closures very much...
$ hg init
$ echo a > a
$ hg ci -qAm 'add a'
$ hg init subrepo
$ echo 'subrepo = http://example.net/libfoo' > .hgsub
$ hg ci -qAm 'added subrepo'
$ hg up -qC 0
$ echo ax > a
$ hg ci -m 'changed a'
created new head
$ hg up -qC 1
$ cd subrepo
$ echo b > b
$ hg add b
$ cd ..
Should fail, since there are added files to subrepo:
$ hg merge
abort: uncommitted changes in subrepository 'subrepo'
[255]