manifest: move manifest creation to a helper function
A future patch will be moving manifest creation to be inside manifestlog as part
of improving our cache guarantees. bundlerepo and unionrepo currently rely on
being able to hook into manifest creation, so let's temporarily move the actual
manifest creation to a helper function for them to intercept.
In the future manifest.manifest() will disappear entirely and this can
disappear.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
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any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).