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 a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3