filemerge: use arbitraryfilectx for backups
With in-memory merge, backup files might be overlayworkingfilectxs stored
in memory. But they could also be real files if the user's backup directory is
outside the working dir.
Rather than have two code paths everywhere, let's use arbitraryfilectx so they
can be consistent.
Differential Revision: https://phab.mercurial-scm.org/D1057
https://bz.mercurial-scm.org/842
$ hg init
$ echo foo > a
$ hg ci -Ama
adding a
$ hg up -r0000
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo bar > a
Should issue new head warning:
$ hg ci -Amb
adding a
created new head
$ hg up -r0000
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo stuffy > a
Should not issue new head warning:
$ hg ci -q -Amc
$ hg up -r0000
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo crap > a
$ hg branch testing
marked working directory as branch testing
(branches are permanent and global, did you want a bookmark?)
Should not issue warning:
$ hg ci -q -Amd