dirstate: add prefix and suffix arguments to backup
This would allow the code explicitly copying dirstate to use this method instead.
Use of this method will increase encapsulation (the dirstate class will be sole
owner of its on-disk storage).
mercurial (__VERSION__) __CODENAME__; urgency=medium
* Automated build performed by upstream.
-- Mercurial Devel <mercurial-devel@selenic.com> __DATE__