transaction: add support for non-append files This adds support for normal, non-append-only files in transactions. For example, .hg/store/fncache and .hg/store/phaseroots should be written as part of the transaction, but are not append only files. This adds a journal.backupfiles along side the normal journal. This tracks which files have been backed up as part of the transaction. transaction.addbackup() creates a backup of the file (using a hardlink), which is later used to recover in the event of the transaction failing. Using a seperate journal allows the repository to still be used by older versions of Mercurial. A future patch will use this functionality and add tests for it.