revset: make tokenize extensible to parse alias declarations and definitions

Before this patch, "tokenize" doesn't recognize the symbol starting
with "$" as a valid one.

This prevents revset alias declarations and definitions from being
parsed with "tokenize", because "$" may be used as the initial letter
of alias arguments.

BTW, the alias argument name doesn't require leading "$" itself, in
fact. But we have to assume that users may use "$" as the initial
letter of argument names in their aliases, because examples in "hg
help revsets" uses such names for a long time.

To make "tokenize" extensible to parse alias declarations and
definitions, this patch introduces optional arguments "syminitletters"
and "symletters". Giving these sets can change the policy of "valid
symbol" in tokenization easily.

This patch keeps original examination of letter validity for
reviewability, even though there is redundant interchanging between
"chr"/"ord" at initialization of "_syminitletters" and "_symletters".
At most 256 times examination (per initialization) is cheaper enough
than revset evaluation itself.

This patch is a part of preparation for parsing alias declarations and
definitions more strictly.

mercurial/revset.py

@@ -129,15 +129,39 b' elements = {'
 
 keywords = set(['and', 'or', 'not'])
 
-def tokenize(program, lookup=None):
+# default set of valid characters for the initial letter of symbols
+_syminitletters = set(c for c in [chr(i) for i in xrange(256)]
+                      if c.isalnum() or c in '._@' or ord(c) > 127)
+
+# default set of valid characters for non-initial letters of symbols
+_symletters = set(c for c in  [chr(i) for i in xrange(256)]
+                  if c.isalnum() or c in '-._/@' or ord(c) > 127)
+
+def tokenize(program, lookup=None, syminitletters=None, symletters=None):
     '''
     Parse a revset statement into a stream of tokens
 
+    ``syminitletters`` is the set of valid characters for the initial
+    letter of symbols.
+
+    By default, character ``c`` is recognized as valid for initial
+    letter of symbols, if ``c.isalnum() or c in '._@' or ord(c) > 127``.
+
+    ``symletters`` is the set of valid characters for non-initial
+    letters of symbols.
+
+    By default, character ``c`` is recognized as valid for non-initial
+    letters of symbols, if ``c.isalnum() or c in '-._/@' or ord(c) > 127``.
+
     Check that @ is a valid unquoted token character (issue3686):
     >>> list(tokenize("@::"))
     [('symbol', '@', 0), ('::', None, 1), ('end', None, 3)]
 
     '''
+    if syminitletters is None:
+        syminitletters = _syminitletters
+    if symletters is None:
+        symletters = _symletters
 
     pos, l = 0, len(program)
     while pos < l:
@@ -177,12 +201,12 b' def tokenize(program, lookup=None):'
             else:
                 raise error.ParseError(_("unterminated string"), s)
         # gather up a symbol/keyword
-        elif c.isalnum() or c in '._@' or ord(c) > 127:
+        elif c in syminitletters:
             s = pos
             pos += 1
             while pos < l: # find end of symbol
                 d = program[pos]
-                if not (d.isalnum() or d in "-._/@" or ord(d) > 127):
+                if d not in symletters:
                     break
                 if d == '.' and program[pos - 1] == '.': # special case for ..
                     pos -= 1