D288: bundlerepo: move temp bundle creation to a separate function

[durham (Durham Goode)]

durham added a comment.


  I fixed the comment and renamed read to readfn. I know we usually do renames as separate from code moves, but this was a tiny rename (only one use).

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D288

To: durham, #hg-reviewers, indygreg
Cc: krbullock, martinvonz, indygreg, lothiraldan, mercurial-devel