bundle2: extract processing of part into its own function
This is code movement only. This will be useful to have it separated for reuse
purposes. We plan to introduce a new feature to the bundle format that allow
inserting a part in the middle of another part payload. This will be useful to
transmit a exception raised during a part generation.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present