context: extract partial nodeid lookup method to scmutil

We will add another caller soon, and there's a non-obvious reason to
use the unfiltered repo that we don't want to copy across the code
base.

Differential Revision: https://phab.mercurial-scm.org/D3189

mercurial/context.py

@@ -450,7 +450,7 b' class changectx(basectx):'
             except KeyError:
                 pass
 
-            self._node = repo.unfiltered().changelog._partialmatch(changeid)
+            self._node = scmutil.resolvepartialhexnodeid(repo, changeid)
             if self._node is not None:
                 self._rev = repo.changelog.rev(self._node)
                 return

mercurial/scmutil.py

@@ -433,6 +433,15 b' def formatrevnode(ui, rev, node):'
         hexfunc = short
     return '%d:%s' % (rev, hexfunc(node))
 
+def resolvepartialhexnodeid(repo, prefix):
+    # Uses unfiltered repo because it's faster when then prefix is ambiguous/
+    # This matches the "shortest" template function.
+    node = repo.unfiltered().changelog._partialmatch(prefix)
+    if node is None:
+        return
+    repo.changelog.rev(node)  # make sure node isn't filtered
+    return node
+
 def isrevsymbol(repo, symbol):
     try:
         revsymbol(repo, symbol)