template: fix shortest(node) function in pure mercurial

Pure mercurial (i.e. without c extensions) does not support partialmatch() on
the revlog index, so we must fall back to use revlog._partialmatch() to handle
that case for us. The tests caught this.

We don't use revlog._partialmatch() for the normal case because it performs a
very expensive index iteration when the string being tested fails to find a
unique result via index.partialmatch(). It does this in order to filter
out hidden revs in hopes of the string being unique amongst non-hidden revs.
For the shortest(node) case, we'd prefer performance over worrying about
hidden revs.

mercurial/templater.py

@@ -368,7 +368,14 b' def shortest(context, mapping, args):'
     cl = mapping['ctx']._repo.changelog
     def isvalid(test):
         try:
-            cl.index.partialmatch(test)
+            try:
+                cl.index.partialmatch(test)
+            except AttributeError:
+                # Pure mercurial doesn't support partialmatch on the index.
+                # Fallback to the slow way.
+                if cl._partialmatch(test) is None:
+                    return False
+
             try:
                 int(test)
                 return False