revsetlang: fix position of '-' in spaceless 'a-b' expressions

I don't know if there is an observable difference, but it seems like
an obvious fix.

Differential Revision: https://phab.mercurial-scm.org/D4410

mercurial/revsetlang.py

@@ -177,7 +177,7 b' def tokenize(program, lookup=None, symin'
                         if p: # possible consecutive -
                             yield ('symbol', p, s)
                         s += len(p)
-                        yield ('-', None, pos)
+                        yield ('-', None, s)
                         s += 1
                     if parts[-1]: # possible trailing -
                         yield ('symbol', parts[-1], s)