filemerge: only write in-memory backup during premerge This wasn't broken, but should mirror the non-in memory case to save an extra write. Differential Revision: https://phab.mercurial-scm.org/D1807

filemerge: fix backing up an in-memory file to a custom location If the user specifies a ui.origbackuppath, we used to always copy the file there, but if the source file is in memory we must write it instead of copying. Differential Revision: https://phab.mercurial-scm.org/D1806