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
os = <unloaded module 'os'>
os.system = <built-in function system>
os = <module 'os' from '?'>
util = <unloaded module 'util'>
util.system = <function system at 0x?>
util = <module 'mercurial.util' from '?'>
util.system = <function system at 0x?>
hgweb = <unloaded module 'hgweb'>
hgweb_mod = <unloaded module 'hgweb_mod'>
hgweb = <module 'mercurial.hgweb' from '?'>
fred = <unloaded module 're'>
remod = <unloaded module 're'>
re = <unloaded module 'sys'>
fred = <unloaded module 're'>
fred.sub = <function sub at 0x?>
fred = <proxied module 're'>
remod = <module 're' from '?'>
re = <unloaded module 'sys'>
re.stderr = <open file '<whatever>', mode 'w' at 0x?>
re = <proxied module 'sys'>
pvecproxy = <unloaded module 'pvec'>
pvecproxy.__doc__ = 'A "pvec" is ...'
pvecproxy.__name__ = 'mercurial.pvec'
pvecproxy.__dict__['__name__'] = 'mercurial.pvec'
pvecproxy = <proxied module 'pvec'>
contextlib = <unloaded module 'contextlib'>
contextlib.unknownattr = ImportError: cannot import name unknownattr
__import__('contextlib', ..., ['unknownattr']) = <module 'contextlib' from '?'>
hasattr(contextlibimp, 'unknownattr') = False
node = <module 'mercurial.node' from '?'>