py3: fix slicing of bisect label in templatefilters.shortbisect()

contrib/python3-whitelist

@@ -18,6 +18,7 b' test-basic.t'
 test-bheads.t
 test-bisect.t
 test-bisect2.t
+test-bisect3.t
 test-blackbox.t
 test-bookmarks-current.t
 test-bookmarks-merge.t

mercurial/templatefilters.py

@@ -349,7 +349,7 b' def shortbisect(label):'
     is not a valid bisection status.
     """
     if label:
-        return label[0].upper()
+        return label[0:1].upper()
     return ' '
 
 @templatefilter('shortdate')