transaction: add a file generation mechanism A new `transaction.addfilegenerator` function is added. It allows external code to register files to be generated. See inline documentation for details. It is important to gather all file creation logic on the transaction as at some point we'll want to mimic the "pre-transaction-commit" logic that we use for revlog. I'm refering to the logic that lets hooks see the result of the transaction before it actually gets committed.

transaction: backup file in a dedicated "namespace" File backup may conflict with other "journal.*" file. We add a fixed part in the backup name file to prevent it.

transaction: use `self.journal` to create backup file Transaction journal name is "journal" in most case, but it can be something else. We use the appropriate attribute to create the file.

memctx: add note about p2

memfilectx: add remove and write methods Similar to the previous patch for workingfilectx, this patch will allow abstracting localrepo.remove / write method to refactor working directory code but instead operate on files in memory.

workingfilectx: add remove and write methods This patch will allow abstracting localrepo.remove / write method to refactor working directory code.

memctx: create a filectxfn if it is not callable This will allow future patches to construct a memctx based on another context or any other store-type object.