storageutil: extract most of emitrevisions() to standalone function As part of implementing a storage backend, I found myself copying most of revlog.emitrevisions(). This code is highly nuanced and it bothered me greatly to be copying such low-level code. This commit extracts the bulk of revlog.emitrevisions() into a new standalone function. In order to make the function generally usable, all "self" function calls that aren't exposed on the ifilestorage interface are passed in via callable arguments. No meaningful behavior should have changed as part of the port. Upcoming commits will tweak behavior to make the code more generically usable. Differential Revision: https://phab.mercurial-scm.org/D4803