revset: allow rev(-1) to indicate null revision (BC)
This can simplify the conversion from numeric revision to string. Without it,
we have to handle -1 specially because repo['-1'] != repo[-1].
The -1 revision is not officially documented, but this change makes sense
assuming that "rev(%d)" exists for scripting or third-party tools.
$ 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