filectx: replace use of _filerev with _filenode
_filerev depends on the filelog implementation using revlogs and linkrevs.
Alternative implementations, like remotefilelog, do not have rev numbers, so
this call fails. Replacing it with _filenode means it doesn't rely on rev
numbers, and doesn't cost anything extra, since _filerev is using _filenode
under the hood anyway.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present