parser: extract helper that creates a dict of aliases

This will be common between revset and templater.

The local variable 'alias' is renamed to 'a' to avoid shadowing the global
'alias' class.

mercurial/parser.py

@@ -462,3 +462,13 b' class basealiasrules(object):'
         if err:
             err = efmt % {'section': cls._section, 'name': name, 'error': err}
         return alias(name, tree, args, err, repl)
+
+    @classmethod
+    def buildmap(cls, items):
+        """Parse a list of alias (name, replacement) pairs into a dict of
+        alias objects"""
+        aliases = {}
+        for decl, defn in items:
+            a = cls.build(decl, defn)
+            aliases[a.name] = a
+        return aliases

mercurial/revset.py

@@ -2320,10 +2320,7 b' def _expandaliases(aliases, tree, expand'
     return result
 
 def findaliases(ui, tree, showwarning=None):
-    aliases = {}
-    for k, v in ui.configitems('revsetalias'):
-        alias = _aliasrules.build(k, v)
-        aliases[alias.name] = alias
+    aliases = _aliasrules.buildmap(ui.configitems('revsetalias'))
     tree = _expandaliases(aliases, tree, [], {})
     if showwarning:
         # warn about problematic (but not referred) aliases