revsetlang: add a getboolean helper function

revsetlang: add a getboolean helper function

This will be used to parse followlines's "descend" argument.

--- a/mercurial/revsetlang.py	Tue Mar 28 10:15:02 2017 +0200
+++ b/mercurial/revsetlang.py	Sat Apr 15 11:26:09 2017 +0200
@@ -199,6 +199,12 @@
     except ValueError:
         raise error.ParseError(err)
 
+def getboolean(x, err):
+    value = util.parsebool(getsymbol(x))
+    if value is not None:
+        return value
+    raise error.ParseError(err)
+
 def getlist(x):
     if not x:
         return []