scmutil: use resolvehexnodeidprefix() from revsymbol()

I should have copied this from changectx.__init__ into in 35b34202dd3b
(context: handle partial nodeids in revsymbol(), 2018-04-08).

Differential Revision: https://phab.mercurial-scm.org/D3310

mercurial/scmutil.py

@@ -503,7 +503,7 b' def revsymbol(repo, symbol):'
         except KeyError:
             pass
 
-        node = repo.unfiltered().changelog._partialmatch(symbol)
+        node = resolvehexnodeidprefix(repo, symbol)
         if node is not None:
             rev = repo.changelog.rev(node)
             return repo[rev]